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Fundam^ 
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FUNDAMENTALS  OF  BOTANY 


GAGER 


THE  LARGEST  AND  OLDEST 
LIVING  THING 

The  General  Sherman  "big  tree"  {Sequoia  gigan- 
tea).  This  tree  was  about  1,200  yrs.  old  when 
Christ  was  born.  At  the  time  of  the  Trojan  wars 
and  the  Exodus  of  the  Hebrews  from  Egypt,  under 
the  leadership  of  Moses,  the  tree  was  a  sapling 
20-30  ft.  high.  It  has  been  alive  during  all  of 
mediaeval  and  modern  history.  Its  height  is 
279.9  ft.,  circumference  of  the  trunk  at  the  base 
102.8  ft.,  greatest  diameter  36.5  ft.,  diameter 
100  ft.  from  the  ground  17.7  ft.  Note  that  its 
lowest  branches  are  at  about  the  level  of  the 
tops  of  the  conifers  surrounding  it.  (Photo  by 
Pillsbury.) 


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mlK^^mrf^f^^^^^^'jHB^'' 

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WsL^^^W^^^^SB^ssBtMi 

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hhW  '  I^r^^^^^^Si^Bi 

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^Rif   ^''^-^^^^^^^^ai 

H  j  M'.'T^  ~  ..j^HSlll^Bfl^S^I 

p|  V^■^^-yi^?^^■-^: 

(Frontispiece) 


FUNDAMENTALS 

OF 

BOTANY 


BY 

C.  STUART  GAGER 

DIRECTOR  OF  THE  BROOKLYN  BOTANIC  GARDEN 


WITH  437  ILLUSTRATIONS 


/S    S 


PHILADELPHIA 

BLAKISTON^S   SON   &   CO 

1012  WALNUT  STREET 


Copyright,  1916,  by  P.  Blakiston's  Son  &  Co. 


PRINTED    IN    U.    S.    A. 
BY   THE    MAPLE    PRESS    COMPANY,    YORK,    PA. 


TO 
M.  K.  G. 

AND 

L.  J.  P. 


PREFACE 


An  introductory  course  of  study  in  botany  should  do  at 
least  six  things  for  the  student: 

1.  Teach  him  the  fundamental  elementary  facts  con- 
cerning plant  life. 

2.  Acquaint  him  with  the  broad,  illuminating  generaliza- 
tions, and  with  the  theories  and  working  hypotheses  which 
have  been  formulated  on  the  solid  basis  of  observed  fact. 
An  intelligent  comprehension  of  these  fundamental  con- 
cepts is  far  more  important  for  purposes  of  general  culture 
and  a  liberal  education,  and  also  for  more  advanced  study, 
than  an  intimate  acquaintance  with  the  facts  alone  on 
which  the  generalizations  are  based. 

3.  Familiarize  him  with  the  methods  of  thought  and 
work  by  means  of  which  the  science  has  been  and  is  being 
advanced. 

4.  Give  him  some  acquaintance  with  the  great  names  in 
the  history  of  the  science,  so  that  he  may  view  our  present 
body  of  knowledge  in  true  perspective.  The  student 
should  be  made  to  realize  that  what  we  now  know  was  not 
obtained  ''ready  made,"  but  only  by  painstaking  investi- 
gation and  search.  Therefore,  brief  references  are  made 
in  the  following  pages  to  a  number  of  pioneers  in  the  un- 
explored fields  of  botanical  discovery.  From  an  educa- 
tional point  of  view  it  is  quite  as  important  to  understand 
that  our  present  body  of  knowledge  has  been  a  gradual 


X  PREFACE 

possession  of  the  human  race,  and  to  know  how  we  came 
to  acquire  it,  as  to  possess  the  information  which  is  our 
rich  heritage. 

5.  Impress  upon  him  the  present  Hmitations  of  our 
knowledge,  and  acquaint  him  with  some  of  the  more 
important  problems  awaiting  solution. 

6.  Enlist  his  interest  in  the  science  by  so  presenting 
it  as  to  make  him  sense  its  importance  and  value  to  human 
life  in  general,  and  to  his  own  life  in  particular,  and  thereby 
stimulate  him  to  pursue  the  study  further,  and,  may  be, 
to  the  point  of  making  his  own  original  contributions.  A 
class  that  is  properly  taught  can  never  inquire,  at  the  end 
of  the  course,  "What  is  the  use  of  knowing  all  that?" 

Outside  of  technical  and  professional  schools,  no  intro- 
ductory course  in  any  subject  should  ever  be  planned  or 
presented  on  the  supposition  that  its  main  purpose  is  to 
pave  the  way  for  more  advanced  courses.  The  main  func- 
tion of  all  introductory  courses  is,  as  the  name  implies,  to 
introduce  the  pupil  to  a  new  realm  of  thought,  to  acquaint 
him  with  a  possible  new  interest  in  life.  If  he  be  led  to 
discover  himself  in  this  new  realm — well  and  good;  if  not 
— well  and  good  also;  he  will  find  himself  elsewhere,  but 
will  always  be  enriched  and  liberalized  from  the  widening 
of  his  mental  horizon  by  contact  with  another  discipline 
than  his  major  interest  in  life. 

In  presenting  an  elementary  course,  the  aim  should  not 
be  to  make  the  subject  simple — to  remove  all  difficulties — 
but  to  make  it  really  interesting,  that  is,  significant  to  the 
pupil  in  his  own  life,  and  to  make  it  as  rich  as  possible  as 
a  revelation  of  those  broad  basic  principles  which  are  fun- 
damental to  all  true  culture,  but  which  are  to  be  amplified 
and  more  deeply  investigated   only  in  more  advanced 


PREFACE  xi 


courses.     The  presentation  should  be  made  as  simple  as 
possible,  consistent  with  the  realization  of  these  aims. 

From  the  standpoint  of  pure  science,  the  most  funda- 
mental problem  of  botany  is  that  of  the  development  of 
the  individual  plant;  the  ultimate  problem  that  of  the 
development  of  the  kingdom  of  plants.  In  other  words 
the  foundation  and  the  ultimate  goal  are,  respectively, 
ontogeny  (hfe-history  of  the  individual)  and  phylogeny 
(life-history  of  the  race). 

Ontogeny  is  fundamental  because  without  a  knowledge 
of  its  processes  the  processes  of  phylogeny  cannot  be  com- 
prehended. Phylogeny  is  the  ultimate  problem  because 
its  complete  solution  involves  an  orderly  description  of  all 
the  phenomena  of  plant  life,  and  their  relation  to  each 
other. 

Thanks  to  the  nature-study  movement,  most  students 
have  nowadays  acquired  some  knowledge  of  the  parts  and 
a  few  of  the  functions  of  a  flowering  plant  before  they  take 
up  the  study  of  formal  botany;  for  such,  Chapter  II  will 
serve  only  as  a  timely  review;  for  others,  as  an  essential 
preparation  for  the  subsequent  chapters  of  Part  I. 

From  an  educational  point  of  view,  the  most  rapid 
progress  and  the  most  substantial  results  are  to  be  ob- 
tained by  an  order  of  topics  so  arranged  that  each  will 
throw  the  greatest  amount  of  light  on  those  that  follow. 
It  is  also  an  immense  gain  for  the  pupil  to  be  introduced 
to  the  broad  generahzations  of  the  science  by  being  led 
to  meet  them  for  the  first  time  in  that  type  where  their 
concrete  embodiment  is  most  clearly  defined  and  most 
easily  discerned.  In  the  author's  mind,  acceptance  of 
these  two  postulates  points  unmistakably  to  the  fern  as, 
par  excellence,  the  best  plant  with  which  to  begin  the  study 


Xll  PREFACE 

of  life-histories.  A  successful  teacher  of  zoology  once 
declared  that  frogs  were  created  for  the  express  purpose 
of  serving  as  the  introductory  type  in  the  study  of  that 
science;  as  strong  a  claim  may  be  made  for  the  fern  in 
botany. 

Such  concepts  as  alternation  of  generations,  sexual  vs. 
asexual  reproduction,  fertiliza<"ion,  heredity,  adjustment 
to  environment,  life-cycle  are  nowhere  more  clearly  illus- 
trated than  in  the  fern;  the  essence  of  them  all  may  be 
clearly  comprehended  by  anyone  who  has  carefully  studied 
its  life-history.  And  with  what  a  rich  equipment  may  the 
Hfe-histories  of  all  other  forms,  both  higher  and  lower,  be 
then  undertaken!  As  Athene  sprang  full  armed  from  the 
imperial  head  of  Zeus,  so,  from  a  study  of  the  fern,  do  all 
the  essentials  of  alternation,  sex,  life-cycle,  et  cetera,  leap 
clearly  defined  into  the  mind  of  the  beginning  student, 
there  to  remain  throughout  the  course,  illuminating  all 
subsequent  studies  of  life-histories. 

During  the  past  fifteen  or  twenty  years  it  has  been  the 
general,  if  not  universal,  custom  to  study  in  the  laboratory 
the  same  forms  as  those  treated  in  the  text.  Part  II  of 
the  present  book  has  been  planned  with  the  idea  of  having, 
for  the  most  part,  different  sets  of  forms  discussed  in  the 
laboratory  and  the  lecture  room.  This  plan  not  only  gives 
the  pupil  acquaintance  with  a  wider  range  of  types,  but 
also  tends  to  insure  greater  independence  in  the  laboratory 
work.  It  has  been  tacitly  assumed  that,  in  connection 
with  the  use  of  this  text,  substantially  the  same  classic 
types  will  be  studied  in  the  laboratory  as  have  formed  the 
basis  of  laboratory  work  for  two  decades.  They  are  not 
only  types  for  which  material  may  be  obtained  with  com- 
parative ease  in  quantity,  but  also  forms  which  have  been 


PREFACE  xiii 


demonstrated  by  twenty  years  of  trial  to  possess  large 
teaching  value.  Thus,  for  example,  we  have  Ascophyllum 
Sphagnum,  Anthoceros,  Cycas,  and  Erythronium  in  the 
text,  to  be  supplemented  by  Fucus,  Polytrichum,  Marchan- 
tia,  Zamia,  and  Trillium  in  the  laboratory.  By  this  plan 
the  laboratory  work  can  never  degenerate  into  merely 
having  the  student  pretend  to  ''verify"  the  statements  in 
the  text. 

It  is  anticipated  that  a  laboratory  guide,  planned  to 
accompany  this  text  and  carry  out  the  idea  just  outlined, 
may  soon  become  available. 

In  the  matter  of  illustrations,  the  author  has  been  most 
fortunate  in  being  able  to  command  the  services  of  Miss 
Maud  H.  Purdy  for  the  preparation  of  all  original  draw- 
ings, and  of  Mr.  Louis  Buhle,  photographer  at  the  Brook- 
lyn Botanic  Garden,  in  making  most  of  the  photographic 
negatives  and  prints  not  otherwise  acknowledged  in  the 
legends.  For  those  so  acknowledged  the  author  expresses 
here  his  best  thanks  to  authors  and  publishers  who  have 
freely  granted  permission  to  reproduce  copyrighted  as 
well  as  uncopyrighted  illustrations.  The  collections  of 
living  plants,  photographs,  and  drawings  at  the  Brooklyn 
Botanic  Garden  have  been  freely  at  the  disposal  of  the 
author,  and  grateful  recognition  is  here  made  to  that  insti- 
tution for  the  exceptional  opportunities  which  it  has 
afforded. 

Special  appreciation  is  here  recorded  for  permission  from 
Prof.  David  M.  Mottier  and  Prof.  Harlan  H.  York  to 
reproduce,  in  advance  of  their  own  publication,  Figs.  8 
and  263,  respectively. 

Specifically,  acknowledgment  is  made  to  authors  and 
publishers,   for  permission  to  reproduce  illustrations  as 


XIV  PREFACE 

follows:  Prof.  W.  L.  Bray  and  the  New  York  State  College 
of  Forestry  (Syracuse  University),  Fig.  266;  Dr.  N. 
L.  Britton  and  Charles  Scribner's  Sons,  Figs.  341  and 
379;  Prof.  D.  H.  Campbell  and  The  Macmillan  Co.,  Figs. 
275,  283,  140,  141,  and  146;  Prof.  F.  E.  Clements  and 
Henry  Holt  and  Co.,  Fig.  27;  Mr.  Frederick  V.  Coville 
and  the  U.  S.  Department  of  Agriculture,  Division  of 
Publications,  Fig.  336;  Prof.  H.  H.  Dixon  and  The 
Macmillan  Co.,  Fig.  35;  Doubleday  Page  &  Co.,  Fig.  225; 
Prof.  H.  Garman  and  the  Kentucky  Agric.  Exp.  Station, 
Fig.  228;  Prof.  William  F.  Ganong  and  Henry  Holt  and 
Co.,  Fig.  69;  Prof.  Patrick  Geddes  and  John  Murray, 
Fig.  240;  Geological  Survey  of  Ohio,  Fig.  581;  Prof.  W. 
D.  Hoyt  and  the  Bureau  of  Fisheries,  Washington,  D.  C, 
Fig.  177;  the  Ohio  Geological  Survey,  Fig.  412;  Prof. 
A.  C.  Seward  and  The  Cambridge  University  Press  (Lon- 
don), Figs.  413  and  414;  Prof.  D.  H.  Scott  and  A.  &  C. 
Black,  Figs.  415  and  418;  Dr.  Albert  Schneider  and 
Willard  N.  Clute  &  Co.,  Fig.  238;  Prof.  Hugo  de  Vries 
and  The  Open  Court  Publishing  Co.,  Figs.  401  and  402; 
Prof.  G.  R.  Wieland  and  the  Carnegie  Institution  of 
Washington,  Figs.  421-427;  Prof.  S.  W.  Williston,  Fig.  432. 

The  conception  of  diagramning  life-cycles,  as  in  Figs. 
321,  329,  and  others,  appears  to  have  originated  with 
Prof.  John  H.  Schaffner,  of  Ohio  State  University.  These 
diagrams  possess  admirable  teaching  value. 

It  is  a  pleasure  for  the  author  to  acknowledge  a  large 
indebtedness  to  his  colleagues  on  the  staff  of  the  Brooklyn 
Botanic  Garden,  and  especially  to  Dr.  E.  W.  Olive,  whose 
careful  and  critical  reading  of  the  entire  manuscript  and 
proof  has  robbed  the  reviewer  of  much  that  would  have 
been  rightfully  his,  and  has  added  much  of  fundamental 


PREFACE  XV 

value.  Special  acknowledgment  is  made  of  Dr.  Olive's 
interest  and  assistance  in  supervising  the  preparation  of 
the  drawing  for  Fig.  198,  on  the  wheat  rust,  and  for  sup- 
plying the  microscopical  preparations  from  which  a 
number  of  illustrations  have  been  made,  as  indicated  in 
the  legends. 

A  similar  valuable  service  was  rendered  by  Dr.  O.  E. 
White  in  reading  and  discussing  the  manuscript  and 
proof  of  Chapters  XXXII-XXXVIII,  and  in  reading 
all  of  the  galley  proof  and  part  of  the  page  proof;  and 
by  Mr.  Norman  Taylor,  in  reading  manuscript  and  proof 
of  Chapters  XXVII-XXX. 

The  author  is  indebted  to  Prof.  E.  C.  Jeffrey,  of  Harvard 
University,  and  Prof.  E.  W.  Berry,  of  the  Johns  Hopkins 
University,  for  suggestions  and  criticisms  in  connection 
with  the  genealogical  trees  (Figs.  433  and  434),  and 
especially  to  Prof.  G.  R.  Wieland,  of  Yale  University,  who 
critically  read  the  manuscript  of  Chapters  XXXVII  and 
XXXVIII,  and  to  whose  stimulating  comments  is  due 
much  of  whatever  value  these  chapters  may  possess. 

For  any  imperfections  and  inaccuracies,  of  whatever 
sort,  in  the  subject  matter,  the  author  alone  is  responsible. 

C.  Stuart  Gager. 

Brooklyn  Botanic  Garden. 
July  19,  1916, 


CONTENTS 


PART  I 
INTRODUCTION 

Chapter  Page 

I.  Fundamental  Notions i 

II.  The  Parts  of  a  Flowering  Plant 7 

III.  The  Cell 14 

PART  II 
THE  VEGETATIVE   FUNCTIONS  OF  PLANTS 

IV.  Loss  of  Water 21 

V.  Absorption  of  Water 47 

VI.  The  Path  of  Liquids  in  the  Plant 61 

VII.  Nutrition 69 

VIII.  Fermentation 94 

IX.  Respiration 103 

X.  Growth 113 

XI.  Adjustment  to  Surroundings 125 

PART  111 

STRUCTURE  AND  LIFE  HISTORIES 

XII.  Life  History  of  a  Fern 147 

XIII.  Life  History  of  a  Fern  (concluded) 168 

XIV.  Fundamental  Principles 179 

XV.  Life  History  of  a  Moss 193 

XVI.  Life  History  of  a  Liverwort 209 

Anthoceros , 212 

Other  Forms 222 

XVII.  Life  Histories  of  Algae 227 

Ascophyllum 227 

xvii 


XVlll  CONTENTS 

Chapter  Page 

XVIII.  Life  Histories  of  Algae  (concluded) 242 

Dictyota  dichotoma 242 

Ulothrix 250 

Pleurococcus 252 

XIX.  Life  Histories  of  Fungi 256 

An  Alga-like  Fungus  (Rhizopus) 256 

A  Sac-fungus  (Microsphaera) 268 

A  " Rust "  Fungus  (Wheat  Rust) 272 

A  Fleshy  Fungus  (Agaricus) 276 

Other  Non-green  Plants 284 

XX.  Economic  Importance  of  Fungi 288 

Fungi  that  cause  Plant  Diseases 290 

Molds 304 

Cold  Storage 305 

Yeasts 306 

Bacteria 306 

Helpful  Bacteria 316 

XXI.  Saprophytism  and  Symbiosis 321 

Saprophytism 322 

Symbiosis 324 

XXII.  The  Problem  of  Sex  in  Plants 344 

XXIII.  From  Alga  to  Fern      362 

XXIV.  Calamites  and  Lycopods 368 

I.  The  Horsetails  (Equisetales) .368 

II.  The  Club-mosses  (Lycopodiales) 376 

III.  Little  Club-mosses  (Selaginellales) 382 

XXV.  Seed-bearing  Plants 390 

The  Cycads 390 

XXVI.  Seed-bearing  Plants  (continued) 412 

Gymnosperms 412 

Life  History  of  the  Pine 412 

Reproduction 416 

XXVII.  Seed-bearing  Plants  (continued) 433 

Life  History  of  an  Angiosperm 433 

XXVIII.  Seed-bearing  Plants  (continued) 446 

Angiosperms 446 

Archichlamydeas 457 

Apetalae 457 

Polypetalae 459 

XXIX.  Seed-bearing  Plants  (continued) 473 

Metachlamydeae  (Sympetalae) 473 


CONTENTS  XIX 

Chapter  Page 

XXX.  Seed-bearing  Plants  (concluded) 489 

Monocotyledons 489 

Types  of  Monocotyledons 491 

XXXI.  Evolution      502 

XXXII.  Darwinism 510 

XXXIII.  Experimental  Evolution 520 

XXXIV.  Heredity 541 

XXXV.  Experimental  Study  of  Heredity 549 

XXXVI.  The  Evolution  of  Plants 569 

XXXVII.  Paleobotany 578 

XXXVIII.  The  Evolution  of  Plants  (concluded) 502 

Appendix 621 


PART  I 
INTRODUCTION 

CHAPTER  I 
FUNDAMENTAL  NOTIONS 

1.  What  is  Botany? — The  names  of  most  sciences  merely 
tell  what  they  are  about.  Thus  the  term  zoology  (from 
the  Greek,  zoon^  animal,  +  logos,  discourse)  indicates 
the  study  of  animals,  geology  the  study  of  the  earth, 
mineralogy  the  study  of  minerals.  A  similar  name  for  the 
study  of  plants  would  be  phytology,  from  the  Greek  phyton 
{(pvTov),  a  plant,  and  this  word  is,  indeed,  sometimes  used. 
But  the  generally  accepted  name,  botany j  tells  more  than 
what  the  science  is  about;  it  points  back  to  why  mankind 
ever  came  to  study  plants.  The  reason  was  because 
plants  are  so  intimately  and  fundamentally  related  to  our 
own  lives  that  it  becomes,  not  only  interesting,  but 
absolutely  essential  to  know  about  them,  and  under- 
stand them. 

The  word  botany  comes  from  a  Greek  word,  hosko 
{^b(TKoy),  meaning,  ''I  eat."  Botany,  then,  was  originally 
the  science  of  things  good  to  eat,  and  the  term  recognizes 
the  fact  that  for  all  of  our  food  we  are  either  directly  or 


D.    a    HILL    LIBRARY 
North  Carolina  State  College 


2  INTRODUCTION 

indirectly  dependent  upon  plants.  The  earliest  "botan- 
ical" interests  were  naturally  in  plants  as  food.  But 
it  must  have  been  discovered  early  in  the  history  of  man- 
kind that  some  plants  were  not  only  not  good  to  eat,  but 
positively  poisonous,  causing  sickness  or  even  death; 
while  others  produced  marked  physiological  effects,  acting 
some  on  one  part  of  the  body,  some  on  another,  like  medi- 
cine, and  thus  was  early  developed  the  study  of  plants 
in  order  to  ascertain  their  medicinal  properties,  and 
their  value  in  the  treatment  of  diseases.  This  interest  is 
still  reflected  in  the  Spanish  name  for  a  drug  store — 
hotica. 

2.  Relation  of  Plants  to  Man. — Thus  we  see  that  the 
primary  reason  for  our  being  interested  in  plants  at  all 
was  because  they  are  intimately  related  to  our  physical 
existence  and  well-being.  As  civilization  advanced, 
other  uses  for  plants  and  plant  products  were  discovered, 
and  thus  other  reasons  for  being  interested  in  them. 
They  furnish  all  the  wood  of  the  world,  and  one  has  only 
to  consider  for  a  moment  how  absolutely  dependent  we 
are  on  wood,  to  realize  still  more  vividly  the  intimate 
relation  between  the  life  of  plants  and  that  of  man  him- 
self. Our  houses  and  furniture  are  of  wood,  our  food 
(the  product  of  plants)  is  shipped  in  wooden  boxes, 
crates,  and  barrels,  over  rails  supported  by  wooden  ties; 
most  of  the  paper  in  use  is  made  of  wood  pulp,  and 
innumerable  articles  in  daily  use — lead  pencils,  tool 
handles,  many  musical  instruments,  et  cetera — are  made 
largely  of  wood.  Surely,  it  would  be  rather  strange 
if  we  did  not  have  some  interest,  at  least,  in  objects 
so  closely  related  to  our  daily  lives,  our  welfare,  and  our 
happiness. 


FUNDAMENTAL  NOTIONS  3 

3.  Relation  of  Botany  to  Other  Sciences. — It  is  not 

possible  to  study  any  one  science  in  disregard  of  all  the 
others.  Plants  are  related  not  only  to  man,  but  to  the 
air  and  soil  in  which  they  live;  their  life  processes 
are  chemical  or  physical  in  nature;  they  are  distributed 
in  space  over  the  earth's  surface,  and  in  time,  in  the 
layers  of  rocks  of  various  geological  ages;  and  so  the 
study  of  botany  touches  meteorology,  chemistry,  physics, 
geography,  climatology,  geology,  the  science  of  soils,  and 
other  branches  of  science. 

4.  Biology. — The  science  which  deals  with  life  in 
general  is  biology,  and  all  the  sciences  which  deal  with 
living  things  are  biological  sciences.  Zoology,  human 
anatomy  and  physiology,  bacteriology,  and  botany  are 
some  (but  not  all)  of  the  biological  sciences,  and  they  are 
all  more  or  less  closely  related  to  each  other.  There  is 
no  hard  and  fast  boundary  line  between  any  of  the 
sciences;  they  represent,  rather,  different  points  of 
view  of  nature.  But  it  is  convenient  to  subdivide  our 
knowledge  more  or  less  arbitrarily  for  purposes  of  study. 

5.  Systematic  Botany. — Just  as  the  various  ''sciences" 
or  ''knowledges"  represent  different  points  of  view  of 
nature,  so  each  science  may  have  subdivisions,  repre- 
senting different  points  of  view  of  its  phenomena.  The 
study  of  plants  for  the  primary  purpose  of  ascertaining 
their  genetic  relationships  is  systematic  botany.  The 
ultimate  aim  of  this  study  is  to  disclose  the  course  of 
the  evolution  of  the  plant  kingdom;  this  aim  can  never 
be  fully  realized,  because  most  of  the  necessary  data 
have  been  lost  forever  in  the  course  of  the  geological 
evolution  of  the  earth.     Systematic  botany  includes: 

(a)  Classification,  or  the  arrangement  of  the  various 


4  INTRODUCTION 

kinds  of  plants  according  to  some  system  (whence  the 
term,  systematic). 

(b)  Taxonomy,  or  the  principles  of  classification,  based 
upon  the  facts  observed  and  their  interpretation. 

(c)  Nomenclature,  the  principles  and  rules  adopted  for 
the  formation  and  assignment  of  plant  names. 

6.  Morphology. — If,  in  our  study,  our  attention  is 
centered  chiefly  on  structure  and  form,  our  point  of  view 
is  that  of  morphology,  and  we  recognize  external  and 
internal  anatomy,  microscopic  anatomy  {histology),  com- 
parative morphology,  experimental  morphology  (which 
attempts  to  ascertain,  by  experiment,  the  causes  of  form 
and  structure),  embryology  (the  study  of  embryos),  and 
other  subdivisions. 

7.  Physiology. — If  we  are  interested  primarily  in  w^hat 
the  various  parts  of  the  plant  are  doing,  rather  than  in  their 
form  and  structure,  our  point  of  view  is  that  of  physiology. 
We  shall  find,  as  we  study,  that  the  facts  of  form  cannot 
be  understood  or  explained  except  in  the  light  of  the 
physiological  work  of  the  given  part;  and  conversely, 
that  physiological  work  cannot  be  explained  unless  the 
structure  is  also  understood. 

8.  Ecology. — Every  plant  lives  in  a  certain  place,  with 
certain  external  surroundings;  in  other  words  it  has  a 
home,  or,  as  we  usually  say,  a  habitat  or  environment. 
In  order  to  live  and  keep  healthy  the  plant  must  be 
favorably  adjusted  to  the  various  features  (factors)  of  its 
environment — the  range  of  temperature,  amount  of  light 
and  moisture,  components  of  the  soil,  the  earth's  at- 
traction (gravity),  and  surrounding  animals  and  other 
plants.  The  science  of  the  relation  of  living  things  to  their 
environment  is  Ecology. 


FUNDAMENTAL  NOTIONS  5 

9.  Plant  Geography. — The  study  of  the  present  dis- 
tribution of  plants  over  the  earth's  surface,  and  of  the 
causes  and  consequences  of  this  distribution,  is  plant 
geography  (sometimes  called  phy  to  geography). 

10.  Fossil  Botany.— The  oldest  known  rocks  contain 
the  remains  of  plants  that  lived  thousands — probably 
millions — of  years  ago.  These  remains  often,  though  not 
always,  of  stone,  are  fossils,  and  their  study  constitutes 
the  study  of  fossil  botany,  or  paleobotany.  This  study 
is  not  only  interesting  in  itself,  throwing  much  light 
upon  our  knowledge  of  plants,  but  is  also  of  great  value 
to  the  geologist,  often  helping  him  to  interpret  correctly 
the  rock-layer,  and  to  decide  to  what  geological  age  it 
belongs.  By  means  of  fossils,  we  may  also  learn  much 
of  the  climate  of  past  ages,  and  the  great  changes  that 
have  since  taken  place.  Thus,  when  we  find  fossil  re- 
mains of  tropical  plants,  such  as  palms,  in  the  rocks  of 
the  present  arctic  regions,  we  know  that  there  must  have 
been  a.  tropical  climate  in  that  latitude  at  the  time  the 
plants,  now  fossils,  were  living  and  growing  there. 

11.  Educational  Value  of  Botany. — From  the  preced- 
ing paragraphs  it  is  evident  that  a  study  of  plants  will  not 
only  give  us  valuable  information  that  may  be  used  to 
advantage  in  every  day  life,  but  that  it  will  give  us  a 
broader  outlook  than  w^e  might  otherwise  obtain  over  the 
past  and  present  of  the  world  in  which  we  live;  it  may 
not  only  suggest  to  us  the  vocation  we  would  prefer  to 
follow,  but  may  give  us  a  breadth  of  view  and  a  wealth 
of  ideas  that  will  help  to  increase  both  our  usefulness  and 
happiness. 

12.  Plan  of  Study. — We  shall  first  review  the  structure  of 
a  familiar  type  of  plant,  and  then  make  an  elementary  study 


6  INTRODUCTION 

of  the  fundamental  life-processes  involved  in  the  nutri- 
tion and  growth  of  the  individual.  The  second  part  of 
the  book  will  be  devoted  to  studying  the  various  kinds 
of  plants,  and  the  different  ways  in  which  they  solve  the 
same  life  problems  of  nutrition  and  reproduction. 


CHAPTER  II 
THE  PARTS  OF  A  FLOWERING  PLANT 

13.  Fundamental  Terms. — The  plants  with  which  we 
are  most  familiar  (flowering  plants)  are  composed  of 
various  parts,  such  as  leaves,  roots,  tendrils;  and  each 


H|i^>  ^^'_y^^ 

Fig.  I. — A  cactus  plant  {Rhipsalis  pachyptera)  with  leafless  stem. 
The  branches  are  flattened,  thus  exposing  more  green  tissue  to  the  light 
in  the  absence  of  leaves. 

part  has  its  special  work  to  perform.  The  various  parts 
of  a  plant  having  their  own  special  work  are  called  organs, 
and  the  special  work  of  an  organ  is  its  function.     Since 

7 


8 


INTRODUCTION 


they  are  composed  of  organs  plants  (and  animals  also) 
are  said  to  be  organized,  and  are  called  organisms.  For  a 
similar  reason,  the  kingdom  of  living  things  is  called  the 
organic  kingdom,  or  the  organic  world. 


Fig.  2. 


-Kohlrabi,  showing  the  stem  modified  as  an  organ  for  the  storage 
of  food,  and  a  well  developed  tap-root. 


14.  Root  and  Shoot. — Everyone  knows  that  the  plants 
with   which   we   are   best  acquainted  have  roots  in   the 


THE  PARTS   OF  A  FLOWERING  PLANT  9 

ground,  and  a  stem  with  leafy  branches  above  ground. 
It  is  well,  however,  to  recall  this  elementary  knowledge 
and  to  get  clear  ideas  of  these  commonly  recognized  parts. 
The  botanist  recognizes  leaves  as  merely  appendages  of 
the  stem  or  branches,  and  branches  as  merely  subdivisions 
of  the  stem.     Stems  may  or  may  not  have  branches  (Figs. 


Fig,  3. — Fibrous  roots  on  cutting  of  sugar  cane. 

I  and  305).  The  stem,  with  its  branches  and  leaves, 
constitutes  the  shoot.  The  shoot,  therefore,  is  all  the 
plant  except  the  roots.  In  broad  outline,  the  structure 
of  any  common  plant  is  made  up  as  follows: 

f  Root    (with    or    without    branches) 
Plant  body  \  (  Stem    (with    or    without    branches) 

Shoot  1  Leaves 


lO 


INTRODUCTION 


16.  The  Root. — When  a  plant  has  more  than  one  root, 
there  may  be  a  main  or  tap-root  with  branches  (Fig.  2), 
or  there  may  be  no  clearly  recognized  main  root,  but 
numerous  roots  of  equal  value,  each  attached  directly  to 
the   stem    (Fig.  3).     The   root-system   frequently  subdi- 


Fig.  4. — Aerial  fibrous  roots  of  the  royal  palm. 

vides  into  smaller  and  smaller  branches  or  rootlets,  and 
these  may  ramify  (branch  and  spread)  extensively  in  all 
directions.  Roots  never  bear  leaves,  but  the  surface  of 
the  finer,  active  roots  is  covered,  for   a  short  distance 


THE  PARTS   OF  A  FLOWERING  PLANT 


II 


back  from  the  tip  with  innumerable  fine  hair-Hke  out- 
growths, root-hairs  (Fig.  36). 

16.  The  Functions  of  Roots. — The  functions  of  roots 
all  have  to  do  with  maintaining  the  life  of  the  individual 
plant  to  which  they  belong,  either  by  holding  the  plant 


I'iG.  5. — Portion  of  root-system  of  a  yellow  birch  {Betiila  lutca),  showing 
roots  serving  to  anchor  the  plant  to  the  substratum.     (Photo  by  Elsie  M 
Kittredge.) 

firmly  fixed  in  the  ground  (anchorage)  (Figs.  4  and  5), 
where  food  elements  are  abundant,  by  taking  in  these  food 
elements  from  the  substratum  (absorption),  or  by  storing 
up,  for  future  use,  food  that  has  been  made  by  the  plant. 


12 


INTRODUCTION 


Fig.  6. 

Fig.  6. — Leaf  of 
a  willow  {Salix  ^p.). 
b,  blade;  p,  petiole; 
s,  stipules. 

Fig.  7. — Diagram 
to  show  the  essential 
parts  of  a  "flower- 
ing" plant.  /.r., 
tap-root ;  b  .r . , 
branch  root;  cot. 
seed-leaf  (cotyle- 
don); i,  internodc; 
a.l,  leaf-axil;  n,  node; 
a.b,  axillary  bud;  r, 
receptacle  of  floral 
organs;  ca,  calyx; 
per,  perianth;  co, 
corolla;  st,  stamens 
(androecium);  pi, 
pistil  (gynoecium). 


Fig.  7. 


THE    PARTS    OF    A    FLOWERING    PLANT  13 

17.  The  Shoot. — As  stated  above,  the  shoot  is  composed 
of  a  branched  or  an  unbranched  stem,  usually,  but  not 
always,  bearing  leaves  (Figs,  i  and  57).  The  leaves  are 
commonly  composed  of  a  flat,  expanded,  and  usually 
green  part  (the  blade),  which  may  or  may  not  be  borne 
on  a  leaf-stalk  {petiole).  The  portion  of  the  leaf  attached 
to  the  stem  is  the  leaf-base,  the  edge  of  the  blade  is  the 
margin,  the  tip  of  the  blade  is  the  apex,  and  the  portion 
of  the  blade  attached  to  the  petiole  is  the  base  of  the  blade 
(Fig.  6).     These  parts  may  be  tabulated  as  follows: 

f  Stem 


Shoot  < 


Branches 

'  Blade 


Leaves 


Petiole 

Leaf-base 

Stipules 


Apex 

Margin 

Base    of    the    blade 

Veins 


The  main  functions  of  leaves  are:  (i)  to  elaborate  plant 
food  in  the  presence  of  sunlight;  (2)  to  help  regulate  the 
water  content  of  the  plant.  In  these  two  functions  hes 
the  significance  of  the  fact  that  the  leaf-blade  is  flat, 
expanded,  thin,  and  green.  This  will  be  explained  in 
Chapters  IV  and  VII.  Leaves  also  have  other  important 
functions,  to  be  mentioned  later. 

The  branches  serve  to  support  the  leaves,  to  hold  them 
up  into  the  hght  and  air,  and  to  connect  them  with  the 
root-system. 

18.  The  Flower.— The  interpretation  of  the  flower  is 
not  essential  at  this  point,  and  is  reserved  for  a  future 
chapter  (Chapter  XXIX),  when  it  may  be  better  under- 
stood. It  is  sufficient  here  to  state  that  the  chief  function 
of  the  flower  is  the  production  of  seed. 

The  essential  parts  of  a  flowering  plant  are  shown  in 
the  diagram  (Fig.  7). 


CHAPTER  III 
THE  CELL 

19.  Historical. — The  advancement  of  our  knowledge 
of  nature  has  often  depended  upon  the  invention  of  some 
new  instrument  that  made  possible  observations  that 
could  not  have  been  made  without  its  aid.  The  balance 
did  this  for  chemistry,  the  telescope  for  astronomy,  the 
thermometer  for  medicine.  The  possibiUties  for  under- 
standing plant  life  were  more  than  doubled  by  the  in- 
vention of  the  compound  microscope.  By  its  aid  the 
study  of  the  finer  internal  structure  of  plants  was  made 
possible. 

20.  Robert  Hooke. — One  of  the  earliest  to  employ  the 
microscope  in  this  way  was  Robert  Hooke  (163  5-1 703) 
of  England.  He  was  at  first  interested  in  demonstrating 
the  powers  of  the  microscope  on  various  objects.  Among 
them  he  tried  thin  sections  of  cork,  and  found  the  cork  to 
be  composed  of  little  compartments,  which  he  called 
cells,  since  they  roughly  resembled  the  cells  of  a  honey- 
comb. Marcello  Malpighi  (1674),  an  Italian,  and  Nehe- 
miah  Grew,  an  EngHshman  (1682),  greatly  extended  the 
microscopic  study  of  plants,  adding  so  much  to  our  knowl- 
edge that  they  are  now  often  referred  to  as  the  fathers  of 
plant  anatomy. 

21.  Protoplasm. — At  first  the  attention  of  botanists 
was  devoted  almost  exclusively  to  the  walls  of  these  cell- 
like compartments,  and  to  their  shape  and  arrangement. 

14 


THE  CELL 


15 

The  walls  were  considered  the  important  feature,  and  the 
term  cell  meant  the  space  enclosed  by  the  wall.  All  this 
was  very  natural,  for  botanists  had  quite  generally,  up 
to  this  time,  devoted  their  best  energies  to  studying  the 
form  and  structure  of  plants,  paying  relatively  little  at- 
tention to  their  hfe  functions,  or  physiology.  Gradually, 
however,  it  came  to  be  recognized  that  the  really  impor- 
tant part  was  the  substance  that  filled  the  little  compart- 
ments in  all  living  tissues}  It  finally  came  to  be  under- 
stood that  this  is  the  only  living  substance  in  plants 
(and  in  animals  as  well),  and  that  the  cell- walls,  and  in 
fact  the  entire  organism,  are  built  up  by  the  activity  of 
this  remarkable  substance.  It  was  first  called  by  several 
different  names,  but  Hugo  Von  Mohl,  a  noted  German 
botanist,  called  it  protoplasm,'^  considering  it  as  the  first 
organic  substance  formed  from  the  inorganic  materials 
taken  in  by  the  plant.  This  name  was  generally  adopted 
by  both  botanists  and  zoologists. 

22.  The  Cell-theory.— The  idea  that  all  living  things 
are  composed  of  cells,  that  the  cell  is  the  unit  of  plant  and 
animal  structure,  and  that  the  essential  thing  about  the 
cell  is  the  protoplasm,  was  elaborated  by  Schleiden  (1838) 
(for  plants)  and  by  Schwann  (1839)  (for  animals),  and 
was  accepted  as  generally  correct  by  all  students  of  plants 
and  animals.  This  doctrine  became  known  as  the  cell- 
theory  of  Schleiden  and  Schwann.  The  term  cell  is  now 
used  in  biology  chiefly  to  designate  the  protoplasm  comprised 
within  the  cell-wall.  A  cell,  then,  is  not  a  compartment 
containing  something,  but  is  a  structural  unit  of  living 

1  An  intimately  connected  layer,  group,  or  body  of  similar  cells,  all 
having  like  functions,  is  a  tissue. 

»  Greek  protos  (first)  +  plasma  (thing  formed). 


i6 


INTRODUCTION 


matter.     Many  biologists   now  use   the   term  protoplast 
(instead  of  cell)  to  designate  the  units  of  protoplasm. 

23.  Structure  of  the  Cell. — Painstaking  microscopic 
study  of  cells  has  revealed  the  fact  that  they  have  a  wonder- 
fully beautiful  and  complex  structure  (Figs.  8  and  9). 
The  protoplast  is  composed  of  two  clearly  defined  parts, 
a  denser,  more  or  less  globular  portion,  the  nucleus,  sur- 
rounded   by    cytoplasm  {i.e.,  cell-plasm).     Nucleus    and 


r^Wl^mk--^^ 


Fig.  8. — Cross-section  of  a  cell  from  the  root  of  a  marrow-fat  pea. 
w",  nucleolus;  n.p  nucleoplasm;  n.m,  nuclear  membrane;  a.m,  starch- 
forming  plastid;  st,  starch  grain;  c.w,  cell-wall;  c.p^  cytoplasm;  ch,  chon- 
driosomes;  they  are  scattered  throughout  the  cytoplasm.  (After  D.  M. 
Mottier.) 


cytoplasm  together  constitute  protoplasm.  The  nucleus 
was  discovered  by  Robert  Brown,  in  1831.  The  sub- 
stance of  the  nucleus  is  designated  nucleoplasm,  and  there 
is  generally  a  still  denser  body  in  the  nucleus — the  nu- 
cleolus (plural,  nucleoli).  Sometimes  there  is  more  than 
one  nucleolus  within  the  nucleus.  The  most  important 
chemical  substance  in  the  nucleus  is  chromatin,  a  very 
complex  protein,  rich  in  phosphorus.  The  name  chromatin 
refers  to  the  dense  color  it  acquires  when  treated  with 


THE  CELL 


17 


certain  stains.  The  cytoplasm  of  many,  possibly  of  all, 
})lant  cells  contains,  scattered  through  it,  numerous  tiny, 
deeply  staining  granules,  called  chondriosomes. 

{a)  The  cytoplasm  appears  net-like,  or  reticular,  in 
structure,  and  the  spaces  between  the  meshes  are  vacuoles. 
Each  vacuole  is  filled  with  an  aqueous  solution  of  various 


Fig.  9. — Diagram  of  a  plant  cell  in  perspective,  with  portions  of  adja- 
cent cells.  Note  the  nucleus.  The  lighter  areas  are  vacuoles  in  the 
cytoplasm. 


substances,  known  as  the  cell-sap.  Insoluble  solids  of 
various  nature,  such  as  crystals  and  starch  grains,  com- 
monly occur  in  the  vacuoles. 

(6)  At  every  free  surface,  such  as  the  outer  surface  and 
the  walls  of  the  vacuoles,  the  cytoplasm  is  specially  organ- 
ized into  a  limiting  membrane  (or  plasma-membrane). 
The  structure  of  this  membrane  is  not  well  understood, 


i8 


INTRODUCTION 


although  it  is  one  of  the  most  important  parts,  and  much 
study  is  now  being  given  to  it,  in  an  endeavor  to  under- 
stand it  better. 


¥ 

r 
I 

k 

^ 

r 

i 

Mi^^^^ 

[ 

■ 

■ 

".•^'»^  v  ^^^^II^^^^^^^^^^^^^Hb 

Fig.  10. — Robert  Brown  (i 773-1858).  One  of  the  greatest  of  English 
botanists.  He  discovered  the  nucleus  in  cells,  and  also  the  gymnospermy 
of  Conifers  and  Cycads. 

{c)  The  structural  elements  of  a  cell  may  be  concisely 
tabulated  as  follows: 


Cell 


Nucleus  (nucleoplasm) 

Nuclear  membrane 

Nucleolus 
Cytoplasm 

Limiting  membrane 

Vacuoles 

Vacuolar  membranes 

Cell-sap 

Other    contents    (inclusions) 
Cell-wall 


THE   CELL  19 

id)  Quite  commonly,  in  plants,  adjacent  protoplasts 
are  joined  together  by  strands  of  cytoplasm  passing 
through  minute  pores  in  the  cell-wall. 

24.  Peculiar  Properties  of  Protoplasm. — More  is  known 
of  the  structure  of  protoplasm  than  is  indicated  above, 
but  a  more  detailed  treatment  is  reserved  until  Chapter  X. 
Quite  as  important  as  the  structure  of  protoplasm  are  the 
physiological  or  functional  characteristics,  or  properties, 
that  distinguish  it  from  every  other  known  substance. 
The  most  significant  and  wonderful  of  these  is  its  ability 
to  reproduce  itself.  By  the  vital  activities  of  animals 
and  plants,  their  living  substance  undergoes  a  continual 
destruction,  which  is  accompanied  by  continual  construc- 
tion. Parts  which  are  destroyed  are  constantly  replaced, 
and  new  protoplasm  is  continually  being  formed.  No 
other  known  substance  can  do  this.  If  a  crystal,  for  ex- 
ample, of  salt,  is  suspended  in  a  saturated  solution  of 
salt  in  water,  some  of  the  salt  particles  in  solution  will 
attach  themselves  to  the  crystal  in  a  regular  manner,  so 
as  to  enlarge  it,  while  preserving  its  characteristic  shape. 
But  here,  as  is  readily  recognized,  the  crystal  itself  is 
entirely  inactive.  It  does  not  change  another  kind  of 
matter  into  salt,  but  merely  serves  as  a  center  of  deposit 
for  more  salt.  Protoplasm,  on  the  other  hand,  entirely 
alters  the  nature  of  the  substances  which  enter  into  it, 
and  recombines  them  into  a  substance  like  itself,  with 
entirely  new  properties — in  fact  converts  the  non-living 
into  the  living. 

25.  Secretions. — In  the  course  of  its  continual  de- 
struction and  reconstruction,  protoplasm  gives  off  or 
secretes  other  substances,  unlike  itself  and  unlike  the 
material    of    which    it    was    formed;    these    are    called 


20  INTRODUCTION 

secretions.  Such  is  the  origin  of  the  cell-wall.  It  is 
secreted  by  the  protoplast  which  it  encloses.  Sugar  (as 
in  the  sugar-cane),  starch  (as  in  corn  and  nearly  all 
plants),  fats  (as  in  Brazil  nuts),  and  green  coloring 
matter  and  other  pigments,  are  among  the  substances 
secreted  by  protoplasm. 

26.  Complexity  of  the  Cell. — A  recent  writer,  after 
describing  the  minute  details  of  cell-structure,  states 
that,  ''The  vital  processes  exhibited  by  a  cell  indicate  a 
complexity  of  organization  and  a  minuteness  in  the 
details  of  its  mechanism  which  transcend  our  compre- 
hension and  bafHe  the  human  imagination,  to  the  same 
extent  as  do  the  immensities  of  the  stellar  universe." 

27.  Value  of  the  Cell-theory. — It  is  hardly  possible  to 
overestimate  the  value  of  the  cell-theory  to  botany,  and 
to  all  biological  science.  By  means  of  it  we  are  led  to 
see  that  all  the  vital  activities  of  any  living  thing  have 
their  seat  in  the  protoplasts  of  the  individual  cells.  If  a 
plant  or  an  animal  grows,  it  does  so  because  the  individual 
cells  of  its  body  multiply  and  grow;  if  it  respires,  it  is 
because  every  living  cell  of  its  body  respires;  if  a  wound 
heals,  it  is  because  the  adjacent  cells  reproduce  them- 
selves and  form  new  tissue  to  replace  that  destroyed  by 
the  wound;  sickness  results  because  certain  cells  behave 
abnormally,  or  perform  their  normal  functions  out  of 
place;  reproduction  is  the  setting  free  by  an  organism  of 
one  or  more  of  its  cells,  which  become  the  starting  point 
of  a  new  individual.  In  fact,  all  that  a  plant  or  animal 
does,  physiologically  speaking,  is  the  sum  total  of  what 
the  cells  that  compose  it  do.  Thus  the  cell-theory  gives  us 
a  necessary,  basic  idea  of  all  life-processes. 


PART  II 
THE  VEGETATIVE  FUNCTIONS  OF  PLANTS 


CHAPTER  IV 
LOSS  OF  WATER 


28.  Plants  are  Alive. — The  most  fundamental  concep- 
tion of  a  plant  is  that  it  is  alive,  just  as  truly  and  in  the 
same  sense  as  is  any  animal.  Therefore  it  takes  in  water 
and  food,  respires,  grows,  moves,  responds  when  stimu- 
lated, reproduces,  grows  old,  and  dies.  We  are  so  ac- 
customed to  associate  life  with  activity  that  one  who, 
for  example,  views  a  large  tree,  especially  in  winter, 
stripped  of  its  leaves,  and  apparently  motionless,  except 
when  swayed  by  the  wind,  is  not  always  conscious  of  the 
fact  that  the  tree  really  is  alive.  A  study  of  p'.ants, 
however,  teaches  us  the  fallacy  of  the  idea  that  life  is 
always  associated  with  evident  motion. 

29.  Kinds  of  Vital  Activity. — Everything  a  plant  does 
affects  either  one  of  two  things — either  (i)  the  main- 
tenance of  the  individual,  or  (2)  the  perpetuation  of 
the  race  to  which  that  individual  belongs.  These  two 
classes  of  functions  are  known,  respectively,  as  (i) 
vegetative  and  (2)  reproductive.  This  and  the  next  five 
chapters  will  deal  with  the  vegetative  functions  of  plants. 

30.  Loss  of  Water  Demonstrated.— If  a  leafy  branch, 
cut  from  any  plant,  is  laid  aside  for  a  time  it  will,  as  is 


22  THE   VEGETATIVE    FUNCTIONS    OF   PLANTS 

well  known,  become  wilted.     Potted  plants,  if  not  kept 


Fig.  II. — A  fern  {Cyrlomium  falcatum),  well  watered  and  turgid. 
(Cf.  Fig.  12). 


Fig.  12. — A  fern  {Cyrtomiiim  Jalcalum),  deprived  of  water  for  48  hours. 
The  same  plant  as  shown  in  Fig.  11. 

well  watered,  and  plants  growing  out  of  doors,  if  not  sup- 
plied with  sufficient  rain,  will  also  wilt  (Figs.  11  and  12). 


LOSS  OF  WATER 


23 


M    C    3   <U   W 


a.5 


■a5  fl  2  a 


.24 


THE   VEGETATIVE    FUNCTIONS    OF   PLANTS 


Some  plants  will  imicli  more  readily  than  others,  hut  in  all 
cases  the  wilting  is  due  to  the  fact  that  water  is  being 
continually  given  off  by  the  plant.  This  may  be  easily 
demonstrated  by  placing  a  living  plant,  or  a  few  fresh 


Fig.  14. — Asicr  sp.,  showing  transition  from  i)etiolate,  through  petio- 
late-stipulate,  to  sessile  leaves.  Note  the  elongation  of  the  lower  petioles 
thus  bringing  the  blade  out  to  better  light  exposure. 


leaves  of  any  convenient  plant,  under  a  bell-jar  or  a 
tumbler,  whose  inner  surface  is  first  known  to  be  per- 
fectly dry.  As  a  check  or  control  on  the  experiment  a 
second  bell-jar  should  be  placed  beside  the  first  one,  but 
without  any  plant  or  leaves  under  it.     The  inner  surface 


LOSS   OF   WATER  2$ 

of  the  jar  will  soon  become  clouded  by  a  thin  film  of 
moisture,  and  within  a  very  short  time,  this  moisture  will 
.begin  to  collect  in  drops  (Fig.  13). 


i    i    i    4     if 


4  4S 


1 


Fig.  15. — Asler  sp.     Series  of  leaves,  all  from  one  plant,  showing  gradual 
transition  from  petiolate  (lower  right  hand  specimen)  to  sessile  leaf. 

The  surface  of  the  control  jar  will  remain  perfectly  dry. 
If  the  experiment  is  set  in  the  sun,  the  result  will  be 
greatly  hastened.     This  loss  of  water  from  within  living 


26 


THE   VEGETATIVE    FUNCTIONS    OF   PLANTS 


plants  is  called  transpiration,  in  recognition  of  the  fact 
that  the  moisture  must  pass  through  the  epidermis. 

31.  Function  and  Structure. — As  stated  in  Chapter  I, 
the  function  of  an  organ  cannot  be  intelligently  discussed 
unless  its  structure  is  understood,  and  vice  versa,  the 
structure   of  any  part  is   without   meaning,   except   as 


Fig.  i6. — Leaf  of  a  banana  {Musa  sapientwn),  showing  enormous 
development  of  the  blade,  and  also  how  easily  a  leaf-blade  may  be  torn 
in  the  absence  of  a  marginal  vein.  This  leaf-blade  {i.e.  without  the 
petiole)  measured  over  15  ft.  long  and  3  ft.  wide,  thus  exposing  over 
45  sq.  ft.  of  green  tissue  to  the  light. 

viewed  in  the  light  of  its  function.  Therefore,  if  we  wish 
to  understand  transpiration  and  the  other  functions  of 
leaves,  we  must  first  ascertain  their  structure.  This  has 
significance  for  us  only  in  the  light  of  their  physiological 
work. 


LOSS  OF  WATER 


27 


32.  External  Anatomy  of  a  Leaf. — We  have  seen 
(Chapter  II)  that  the  three  main  parts  of  a  leaf  are  the 
blade,  petiole  and  leaf-base.  These  parts  may  manifest 
every  conceivable  variation  as  to  shape  and  size,  and  bear 
every  relation  to  each  other  as  to  proportion.     The  petiole 


Fig.  17. — ^Leaf  of  Hercules  club  {Aralia  spinosa),  partly  thrice  compound. 
The  leaf-blade  measured  15  in.  wide  at  the  base,  and  12  in.  long. 


maybe  more  or  less  shortened,  or  it  may  be  entirely  wanting 
so  as  to  make  the  leaf-blade  sessile  (seated)  on  the  stem  (Figs. 
14  and  15).  The  blade  may  be  greatly  enlarged  (Fig.  16), 
or  more  or  less  branched  (Fig.  17),  or  it  may  be  greatly  re- 
duced, or  even  entirely  wanting.     In  the  latter  case,  the 


28 


THE   VEGETATrV^E    FUNCTIONS    OF   PLANTS 


Fig.  iS. — New  Zealand  raspberry  (Rubus  auslralis).  The  lower  portion 
is  the  petiole;  the  blade  is  reduced  to  three  spiny  veins  slightly  expanded 
at  the  tip. 


Fig.  19.— Homology  of  bud-scales  in  an  ash  {Fraxinus  sp.).  Series 
showing  gradual  transition  from  outer  bud-scale  (at  left)  to  unexpanded 
foliage  leaf  (at  right).     The  outer  bud-scale  is  morphologically  a  leaf-base. 


LOSS   OF   WATER 


29 


petiole  may  take  on  the  character  of  the  blade,  and  perform 
all  its  functions,  as  in  the  case  of  various  acacias.     In  certain 


Fig.  20. — Tulip  bulb;  longitudinal  section.  F,  solid  stem;  B,  flower 
bud;  S,  leaf-bases  serving  as  bud-scales,  and  also  for  the  storage  of  plant 
food. 

leaves  nothing  remains  but  base,  petiole,  larger  veins,  and 
the  tips  of  the  blade,  as  in  the  New  Zealand  raspberry 


IP^I 

^n^^^^^^^^P^  1 

■ 

Iv'^^H 

p*  ^^^^^^p^  1 

H 

^^.^^P^^H 

MJiflH^^I 

M^M 

^^L'   1 

Ih 

i^l 

1 

Fig.  21. — Buds  of  the  tulip-tree  {Llriodeiidroti  tulipifera),  showing  stipules 
as  bud-scales. 


(Fig.  18).     In  some  plants,  as  for  example,  the  grasses 
(Fig.  22),  there  is  no  distinction  between  petiole  and  blade. 


30 


THE   VEGETATIVE    FUNCTIONS    OF    PLANTS 


Leaves  may  be  so  altered  as  to  cease  to  be  foliage  leaves, 
and  serve,  for  example,  as  bud-scales  (Fig.  19),  or  other 
organs.  In  some  bud-scales  both  petiole  and  blade  may 
be  wanting,  the  leaf-base  alone  serving  as  the  scale  (Fig, 
20).     Again,  the  bud-scale  may  consist  chiefly  of  petiole. 


Fig.  22. — Various  types  of  leaf-blade. 


or  of  blade,  or  (as  in  the  tulip-tree)  of  stipules  only  (Fig. 
21).     Variations  of  leaves  are  illustrated  in  Fig.  22. 

It  is  not  essential  here  to  endeavor  to  frame  a  definition 
of  a  leaf,  though  this  would  be  a  profitable  exercise  for 
the  reader.  The  main  thing  is  to  emphasize  the  fact  that 
the  leaf  is  an  exceedingly  plastic  organ,  that  is,  appearing 


LOSS   OF   WATER 


31 


in  many  forms  and  disguises,  and  readily  adjusting  itself 
to  wide  variations  in  its  surroundings.  By  virtue  of  this 
characteristic,  it  helps  to  enable  the  plant  as  a  whole  to 
become  adapted  to  its  surroundings.  For  the  present 
purpose,  we  are  primarily  interested  in  leaves  as  foliage. 

33.  Internal  Anatomy  of  the  Leaf-blade. — If  we  take 
any  convenient  foliage-leaf,  such  as,  for  example,  a  leaf 


g;---w-jigl!!-',. 


— '-^^■^^"■-^-i^^ 


:*$iSl 


Fig.  23. — ^Leaf  of  a  live-for- 
ever {Sedum  sp.),  with  a  por- 
tion of  the  epidermis  peeled 
back.  Underneath  the  epi- 
dermis is  the  mesophyll. 


Fig.  24. — Mullein  {Verbascum  Thap- 
siis).  L,  cross-section  of  leaf-blade, 
showing  relative  thickness  of  layer  of 
epidermal  hairs;  H,  a  single  hair  from  a 
leaf  (greatly  magnified). 


of  the  common  lilac,  we  may  readily  demonstrate,  with 
the  aid  of  a  scalpel  or  sharp  knife,  that  the  surfaces  of 
the  blade  are  covered  with  a  thin  skin  or  epidermis, 
which  may  be  peeled  off  (Fig.  23),  disclosing  the  mid-leaf 
substance  {mesophyll) ,  lying  between  the  upper  and  the 
lower  epidermis.     In  many  leaves  (for  example,  those  of 


32 


THE    VEGETATIVE    FUNCTIONS    OF    PLANTS 


the  great  mullein),  there  are  numerous  more  or  less  promi- 
nent ''hairs/'  which  are  outgrowths  of  the  epidermis,  and 
readily  come  away  with  it  when  it  is  peeled  off  (Fig.  24). 
We  notice  that  the  veins  of  the  leaf  appear  to  be  imbedded 
in  the  mesophyll,  and  that  they  lie  somewhat  nearer  the 
lower  than  the  upper  surface.  The  lower  surface  is  also 
seen  to  be  usually  of  a  lighter  green  color  than  the  upper, 
and  the  lower  epidermis  may  be  peeled  off  more  readily 
than  the  upper.  The  explanation  of  these  facts  is  found 
in  the  microscopic  structure  (histology)  of  the  mesophyll, 
soon  to  be  studied. 

34.  Histology  of  the  Leaf -epidermis. — When   a   small 
portion  of   the  lower  epidermis  is  examined    under    the 


Fig.  25. — ^Lizard's  tail  {Saururus  cerniiiis).  Portions  of  leaf-epidermis; 
U ,  upper  epidermis;  L,  lower  epidermis;  ep,  epidermal  cell;  st,  guard-cells 
of  the  stomata.     (Camera  lucida  drawing.) 


microscope  it  is  seen  to  be  composed  of  larger  cells, 
irregular  in  shape,  and  of  smaller  cells,  usually  somewhat 
half-moon  shaped,  occurring  in  pairs,  and  the  pairs 
irregularly  distributed  at  frequent  intervals  among  the 
larger  cells  (Fig.  25).     The  latter  possess  no  green  color- 


LOSS  OF  WATER 


33 


ing  matter,  but  the  smaller  cells  contain  numerous  green 
bodies.  The  green  substance  is  chlorophyll  (leaf -green) , 
and  the  separate  green  particles  are  chlorophyll-bodies,  or 
chloroplasts.  Between  each  pair  of  smaller  cells  is  a  tiny 
hole  or  stoma  (from  a  Greek  word  meaning  mouth  or 
opening),  and  the  two  cells  are  the  guard-cells  of  the 
stoma  (plural,  stomata)  (Fig.  26). 

The  structure  of  the  upper  epidermis  of  the  same  leaf 
(Fig.  23)  is  seen  to  be  quite  similar  to  that  of  the  lower, 


Fig.  26. — Photomicrograph    of   stomata   from   a  leaf  of  Verbena  ciltata, 
showing  their  condition  at  9  a.m.     (After  Lloyd.) 


except  that  in  most  plants  there  are  fewer  stomata  in  the 
upper  than  in  the  lower  epidermis.  In  some  leaves  {e.g.y 
barberry,  osage  orange,  lilac)  there  are  no  stomata  in  the 
upper  epidermis;  while  in  other  plants,  such  as,  for  ex- 
ample, the  water-lily  whose  leaves  float  on  water, 
there  are  stomata  in  the  upper  epidermis  but  none  in  the 
lower. 

35.  Microscopic  Structure  in  Cross-section. — Since  all 
objects  examined  with  the  aid  of  a  microscope  are  observed 
with  transmitted  light,  that  is,  by  Hght  that  passes  through 


34 


THE  VEGETATIVE   FUNCTIONS    OF   PLANTS 


them  to  the  eye,^  in  order  to  examine  opaque  objects, 
sections  of  them  must  be  cut,  thin  enough  to  be  readily 
transparent.  The  conditions  of  observation  are  also 
much  simplified  by  this  means. 

Thin  cross-sections  of  leaves,  that  is,  sections  cut  at 
right  angles  to  the  surface,  are  readily  made  with  a  sharp 


Fig.  27. — Cross-sections  of  leaves  of  an  oak  {Quercus  novimexicana), 
showing  the  effect  of  different  light  conditions  on  the  internal  anatomy. 
I,  from  leaf  growing  in  sunlight;  2,  from  leaf  growing  in  the  shade.  (After 
Clements.) 


razor.  When  examined  with  the  microscope,  such  sections 
disclose  a  structure  similar  to  that  illustrated  in  Fig.  27. 
The  epidermis,  both  upper  and  lower,  is  seen  to  consist 
of  a  single  layer  of  cells.  The  free  surface  of  the  outer 
cell-wall  is  coated  with  a  layer  of  a  wax-like  substance, 

^  Objects  examined  with  the  unaided  eye  are  observed  with  light  re- 
fluted  from  their  surface  to  the  eye. 


LOSS  OF  WATER  35 

known  as  cuticle.  The  mesophyll,  or  leaf -parenchyma, 
has  two  well-defined  regions.  In  the  portion  next  the 
upper  epidermis  the  cells  are  elongated,  and  arranged  close 
together  at  right  angles  to  the  epidermis.  This  portion  is 
the  palisade  layer.  In  the  other  portion  of  the  mesophyll 
the  cells  are  of  irregular  shapes,  and  loosely  arranged,  with 
intercellular  spaces.  Commonly,  also,  the  palisade  cells 
contain  more,  and  more  deeply  colored,  chlorophyll  grains 
than  do  the  cells  of  the  spongy  parenchyma.  The  above 
facts  make  it  clear  why  the  upper  surface  of  leaves  is 
darker  green  than  the  lower  surface.  Cross-sections  of 
veins  are  also  seen,  imbedded  in  the  spongy  parenchyma. 
Details  of  their  structure  need  not  be  considered  here. 
The  absence  of  chlorophyll  from  the  epidermal  cells 
(except  the  guard-cells)  may  also  be  noted. 

36.  Stomata  and  Guard-cells. — In  the  lower  epidermis, 
sections  of  the  stomata  are  found,  and  it  is  readily  seen 
that  the  stomata  are  tiny  holes  or  pores  through  the  epi- 
dermis, connecting  the  intercellular  spaces  with  the  out- 
side air.  The  guard-cells  are  so  constructed  that  under 
changing  conditions  of  light  and  moisture  they  may  be- 
come more  or  less  turgid.  When  they  become  more 
turgid,  they  are  more  convex,  and  thus  enlarge  the  diam- 
eter of  the  stoma;  when  less  turgid,  they  become  less 
convex  and  this  diminishes  the  size  of  the  opening,  and 
in  certain  cases  may  even  close  it  completely.  By  these 
changes  the  passage  of  water-vapor  or  other  gases  through 
the  stomata  may  be  either  facilitated  or  retarded. 

37.  Structure  of  the  Petiole. — The  main  function  of 
the  petiole  is  to  hold  the  leaf-blade  well  exposed  to  Hght, 
while,  at  the  same  time,  keeping  it  connected  with  the 
stem.     It  will  have  been  noted  already  that  the  veins 


36 


THE  VEGETATIVE    FUNCTIONS    OF   PLANTS 


converge  at  the  ])ase  of  the  blade  (P'ig.  22).  They  may  be 
traced  from  tl\is  point,  through  the  petiole,  into  the 
branch.     The  veins  are  composed  of  fibers  and  vessels, 


Fig.  28. — Horse-chestnut  {Aescidus  Hippocastanmn).  Is,  leaf-scar, 
showing  scars  of  seven  fibro-vascular  bundles,  corresponding,  in  number, 
to  the  seven  leaflets  of  the  compound  leaf,  formerly  attached  at  Is.  The 
leaf  is  drawn  to  a  smaller  scale  than  the  branch. 


closely  associated,  and  are,  therefore,  called  fibro-vascular 
bundles.  A  cross-section  of  the  petiole  of  a  horse-chestnut 
leaf,  for  example,  shows  one  fibro-vascular  bundle  for 
each  leaflet  of  the  compound  blade  (Fig.  28) ;  each  of  the 


LOSS   OF   WATER 


37 


seven  bundles  extends  out  into  the  blade  as  the  prominent 
mid-veifi  of  a  leaflet.  In  a  common  "trick"  of  child- 
hood, the  epidermis  of  the  petiole  of  the  common,  broad- 
leaved  plantain  is  broken  by  sharply  bending  the  petiole, 
or  by  carefully  cutting  with  a 
knife.  The  petiole  may  then  be 
carefully  pulled  apart,  so  as  to  dis- 
close the  fibro-vascular  bundles 
without  breaking  them  (Fig.  29). 
These  bundles  are  the  channels 
through  which  liquids  pass  between 
the  leaf-blade  and  the  branch. 

38.  Transpiration. — In  order  to 
understand  transpiration,  we 
should  have  in  mind  a  clear  pic- 
ture of  the  conditions  within  a 
leaf.^  Because  of  moisture  in  the 
cells,  the  cell- walls  are  saturated. 
From  their  moist  surfaces  water  is 
continually  evaporating  into  the 
intercellular  spaces  (Fig.  27),  so 
that  the  air  in  those  spaces  is  al- 
ways nearly  saturated;  that  is,  it 
holds  nearly  as  much  water  as  pos- 
sible in  the  form  of  vapor.  From 
the  intercellular  spaces  the  vapor 
diffuses  out  through  the  stomata, 

and  passes  off  into  the  air.     If  the  outer  air  is  also  very 
humid,  as  frequently  near  the  ground  after  sunset,  the 


Fig.  29. — Leaf  of  plantain 
{Plantago),  with  the  petiole 
stretched  lengthwise  from  a 
transverse  cut,  showing  the 
fibro-vascular  bundles  that 
continue  up  into  the  five 
main  veins  of  the  leaf-blade. 


^  While  loss  of  water  is  not  confined  to  leaves,  they  are  the  chief  organs 
of  transpiration,  and  if  we  understand  the  process  in  them,  we  shall 
understand  it  elsewhere. 


38 


THE  VEGETATIVE   FUNCTIONS    OF   PLANTS 


vapor  from  within  the  leaves  may  not  be  able  readily  to 
pass  off,  and  will  accumulate  in  drops  on  the  surface 
of  the  leaves,  forming  dew.  The  passing  off  of  water  is 
not  confined  to  the  stomata  {stomatal  transpiration), 
but  may  take  place  through  portions  of  the  epidermis 
where  there  are  no  stomata,  the  water  passing  through  the 
cuticle  {cuticular  transpiration). 

39.  Control  of  Transpiration. — The  rate  of  transpiration 
is  controlled  by   both  external  and  internal  factors.     If 


Fig.  30. — Gasteria     nigricans.     Succulent    leaves,    with    thick    cuticle 
serving  for  the  storage  of  water. 


the  outer  air  is  very  humid,  water  cannot  evaporate  into 
it  as  rapidly  as  when  it  is  less  humid.  On  humid  days, 
therefore,  transpiration  will  be  diminished.  It  is  in 
recognition  of  this  fact  that  gardeners,  in  ''wetting  down" 
a  plant  house,   do  not  confine  the  water  to  the  plants. 


LOSS  OF  WATER 


39 


but  thoroughly  wet  the  walks  and  walls  so  as  to  maintain 
a  favorable  humidity  of  the  air. 

Naturally,  anything  that  tends  to  increase  humidity 
will  retard  transpiration.  If  the  air  in  the  vicinity  of 
foliage  is  quiet,  its  humidity  will  increase  owing  tQ  the 


Fig.  31. — Transpiration  from  four  leaves  of  oleander.  At  the  left, 
both  sides  coated  with  cocoa  butter;  next,  under  surface,  only,  coated; 
next,  upper  surface,  only,  coated;  right,  uncoated.  All  exposed  for  the 
same  length  of  time, 

water-vapor  from  the  leaves,  but  when  the  wind  blows, 
fresh  portions  of  less  humid  air  are  continually  brought 
into  contact  with  the  plants,  and  transpiration  becomes 
more  rapid.  This  is  why  a  plant  or  a  bouquet,  being 
carried  from  one  place  to  another,  will  keep  fresh  longer  if 
wrapped  with  paper. 


40       THE  VEGETATIVE  FUNCTIONS  OF  PLANTS 

Warm  air  can  contain  more  water- vapor  in  a  given  space 
than  colder  air.  For  this  reason,  other  things  being 
equal,  plants  on  which  the  sun  is  shining  will  transpire 
more  rapidly  than  those  in  the  shade,  or  than  on  a  cool, 
cloudy  day.  Florists  take  advantage  of  this  fact  by 
keeping  cut  flowers  in  a  place  artificially  cooled  by  ice. 

Certain  structural  features  of  the  plant  operate  to 
reduce  transpiration.  The  epidermal  hairs,  as  for  ex- 
ample on  the  mullein  leaf,  tend  to  retain  the  more  humid 
air  near  the  surface  of  the  leaf,  even  when  the  wind 
blows.  In  some  plants  {e.g.,  the  tropical  gasterias,  Fig. 
30)  the  cuticle  is  greatly  thickened,  so  that  water  can 
pass  off  only  very  slowly.  The  very  curling  of  leaves, 
when  they  begin  to  wilt,  also  tends  to  reduce  transpira- 
tion by  reducing  the  amount  of  surface  exposed  (Fig.  31). 
The  arrangement  of  leaves  in  a  compact  rosette  accom- 
plishes the  same  result  (Fig.  32). 

Evidence  obtained  by  recent  studies  of  transpiration 
in  several  different  species  of  flowering  plants  indicates 
that  there  is  no  necessary  nor  uniform  relation  between 
the  amount  of  transpiration  and  the  number  of  stomata 
per  unit  of  leaf-surface,  nor  between  the  amount  of 
transpiration  and  the  total  area  of  the  stomata.  These 
studies  indicate  that,  contrary  to  our  earlier  conceptions, 
the  amount  of  transpiration  is  probably  regulated  by  a 
complex  of  several  factors,  among  which  the  stomata  are 
less  important  than  was  formerly  supposed. 

40.  Advantages  of  Transpiration. — It  might  seem,  at 
first  thought,  that  the  loss  of  water  by  transpiration  is  a 
disadvantage  to  plants.  Of  course  this  would  be  the  case 
were  it  not  possible  for  the  roots  to  take  in  water  as  fast 
as  it  is  lost.     When  this  is  not  possible,   transpiration 


LOSS   OF   WATER 


41 


becomes  a  real  source  of  danger  to  the  plant.  Indirectly, 
however,  transpiration  performs  a  great  service,  for  it 
aids  in,  and  is  probably  one  of  the  chief  causes  of,  the 
ascent  of  liquids  taken  in  from  the  soil.  Were  water 
never  given  off,  (either  by  transpiration  or  by  secretion  or 


Fig.  32. — Sempervivum  tabidcBJorme.  The  arrangement  of  the  leaves 
in  a  compact  rosette,  the  hairs  on  their  margins,  their  thick  cuticle,  and 
other  characters,  make  the  plant  xerophytic  or  drought-resistant. 

both,  see  paragraph  41)  it  would  not  be  possible  for  tissues, 
already  turgid,  to  receive  a  fresh  supply,  and,  since  all  the 
elements  of  plant-food  can  be  carried  through  the  plant 
only  in  solution,  the  importance  of  this  point  can  hardly 
be  overestimated. 

The  manner  in  which  transpiration  may  facilitate  the 
passage  of  liquids  through  the  stem  may  be  illustrated 


42 


THE   VEGETATIVE    FUNCTIONS    OF   PLANTS 


by  a  very  simple  experiment.     A  small  leafy  branch  of 
any  plant   (a  branch  of  some  evergreen  is  excellent  to 


Fig.  ;^:^. — Experiment  to  illustrate  the  so-called  "lifting  power"  of 
transpiration,  d,  dish  of  mercury;  mt,  surface  of  mercury  column  that 
has  risen  in  the  glass  tube,  gt;  rt,  rubber  tube;  br.  branch  of  a  pine  tree, 
whose  leaves  are  transpiring. 

use)  is  inserted  in  the  end  of  a  piece  of  glass  tubing  about 
3  feet  long.     The  joint  between  the  glass  and  the  branch 


LOSS   OF   WATER  43 

should  be  made  perfectly  air-tight  by  means  of  a  piece  of 
rubber  tubing,  about  i  inch  long,  extending  over  the  end 
of  the  glass  tube.  Fill  the  glass  tube  with  water,  then 
invert  it,  and  place  the  lower  end  in  a  dish  of  mercury, 
having  care  that  the  water  remains  up  in  the  tube  far 
enough  to  cover  the  end  of  the  branch  (Fig.  2>3)-  As 
transpiration  proceeds,  the  pressure  of  the  atmosphere 
will  force  the  mercury  up  the  glass  tube  as  rapidly  as  the 
water  passes  into  the  plant.  This  experiment  is  some- 
times said  to  illustrate  the  ** lifting  power"  of  transpira- 
tion, but  from  the  explanation  here  given,  it  is  seen  that 
the  mercury  is  not  hfted,  but  pushed  up  the  tube  by  the 
pressure  of  the  outside  air.  This  experiment  should  not 
be  regarded  as  illustrating  more  that  it  really  does;  it 
does  not,  for  example,  explain  the  rise  of  sap  in  plants. 

41.  Ascent  of  Sap. — It  is  a  well-known  fact  that, 
although  living  leaves  deprived  of  water  merely  become 
wilted,  dead  leaves  eventually  dry  up;  they  cannot  supply 
themselves  with  water,  although  evaporation  is  taking 
place  from  their  surfaces,  and  although  the  stem  to  which 
they  are  attached  is  abundantly  supplied.  We  must 
conclude,  therefore,  that  merely  physical  forces  (imbibi- 
tion and  evaporation)  are  not  sufficient  to  account  for 
the  rise  of  liquids  in  stems.  Recent  experiments  indicate 
that,  in  this  connection,  much  importance  should  be 
attached  to  the  secretion  of  substances  by  the  leaf — a 
physiological  process. 

We  are  familiar  with  such  action  in  the  secretion  of 
nectar  by  the  nectar-glands  of  flowers  (Fig.  34).  Some 
leaves  (e.g.,  Colocasia  aniiquorum)  also  secrete  water  so 
rapidly  that  it  falls  in  drops  from  their  tips.  It  is 
probable    that,  in  transpiration,  the  protoplasm   in   the 


44  'J^HE   VEGETATIVE    FUNCTIONS    OF   PLANTS 

mesophyll  cells  of  leaves  first  secretes  a  solution  to  the 
outer  surface  of  the  cells,  and  that  this  solution  becomes 
concentrated  by  evaporation  of  the  liquid  solvent  into 
the  intercellular  spaces.  The  concentrated  solution,  then, 
by  osmotic  action,  causes  more  water  to  be  withdrawn 
from   the   interior  of  the   cells,  and  these,  in  turn,   are 


Fig.  34. — Petal  of  crown  imperial  {Frilillaria  imperialis),  showing  the 
remarkably  large  drop  of  nectar  (n),  secreted  by  the  nectar  gland  near  the 
base  of  the  petal. 

replenished  from  the  fibro-vascular  bundles.  On  account 
of  the  tensile  strength  of  the  water  column  in  these  bundles 
the  water  is  raised  as  rapidly  as  it  is  transpired  from  the 
leaves. 

Thus,  while  the  physical  processes  of  osmosis  and 
transpiration  may  be  factors  in  causing  the  ascent  of 


LOSS   OF  WATER 


45 


sap,  the  physiological  process  of  secretion  is  also  of  very 
great  importance.  Moreover,  on  this  basis  we  are  able 
to  account  for  the  ascent  of  sap  in  submerged  aquatic 
plants,  like  eel-grass,  pond-weed,  and  others,  where  tran- 
spiration is  not  possible,  or  in  land  plants  in  very  humid 
tropical  regions  where  the  nearly  or  quite  saturated  air 
greatly  reduces  transpiration  or  even  wholly  prevents  it 
for  extended  periods  of  time.  In  fact  it  may  be  shown, 
experimentally,  that  a  leafy  branch  can  raise  water  through 


Fig.  35. — Experiment  to  show  that  secretory  action  in  the  cells  of  a 
leaf  are  able  to  cause  the  rise  of  liquid  in  a  branch,  when  evaporation  from 
the  leaf-surfaces  is  impossible,  i,  Beaker  containing  solution  of  eosin; 
2,  cork;  3,  inverted  glass  bell-jar  containing  water;  4,  iron  support.  In 
this  experiment  the  eosin  rose  rapidly  in  the  branch.  (Modified  from 
H.  H.  Dixon.) 


the  fibro-vascular  bundles,  even  when  submerged.  The 
apparatus  is  set  up  as  shown  in  Fig.  35,  where  the  leafy 
branch,  immersed  in  water  in  an  inverted  glass  bell-jar, 
has  the  cut  end  of  the  stem  in  a  solution  of  eosin  or  red 


46  THE  VEGETATIVE    FUNCTIONS    OF    PLANTS 

ink.  Under  these  conditions  only  secretion  can  operate 
to  withdraw  water  from  the  fibro-vascular  bundles,  and 
yet  the  eosin  will  rise  in  the  branch  and  into  the  leaves. 
From  this  and  other  experiments  (not  described  here) 
it  is  evident  that  the  withdrawal  of  water  from  the  fibro- 
vascular  bundles  in  the  stem  by  the  combined  action  of 
secretion,  osmosis,  and  transpiration  may  account  for 
the  ascent  of  sap,  even  to  the  tops  of  very  tall  trees  and 
vines.  These  processes  are  able  to  raise  the  water  column 
on  account  of  its  great  tensile  strength,  by  which  it  does 
not  separate,  although  gravity  pulls  down  on  its  lower 
end,  and  the  physiological  processes  in  the  leaves  result 
in  a  pull  at  its  upper  end. 


CHAPTER  V 
ABSORPTION  OF  WATER 

42.  Importance  of  Water. — Everyone  knows  that  plants 
must  have  a  suitable  amount  of  water  in  order  to  live  and 
keep  healthy.  Deprived  of  water  they  wilt,  ^nd  finally 
die.  If  they  are  given  too  much  water  they  also  suffer. 
The  water  serves  many  purposes.  In  the  first  place,  it 
is  needed  to  keep  the  protoplasm  sufficiently  moist.  Pro- 
toplasm may  keep  alive  though  very  dry,  as  in  the  case 
of  dry  seeds,  but  in  order  to  be  most  active  it  must  have 
enough  water  to  keep  it  in  a  semi-fluid  condition.  In 
the  second  place,  were  it  not  for  water,  no  food  materials 
could  reach  the  protoplasts,  for  there  are,  in  general,  no 
openings  in  the  cell-walls  large  enough  for  solid  matter  to 
pass  through.  Therefore,  all  substances  must  reach  the 
protoplasts  in  aqueous  solution.  Again,  water  is  neces- 
sary in  the  transportation  of  materials  from  one  part  of 
a  plant  to  another;  and  finally,  it  is  necessary  in  order  to 
keep  plants  from  wilting,  for  no  plant  can  live  if  it  is 
permanently  wilted. 

43.  Relative  Water  Requirement. — The  amount  of 
water  required  by  various  kinds  of  plants  in  order  to 
reach  maturity  and  produce  seeds  varies  greatly.  It 
depends  in  part  upon  the  weather  conditions  {e.g.j  sun- 
shine, wind,  humidity,  and  other  factors),  in  part  upon 
food  supply,  and  in  part  upon  the  species  or  variety  of 
plant.     Some  species  are  so  constructed  that  they  con- 

47 


48 


THE   VEGETATIVE    FUNCTIONS    OF    PLANTS 


serve  more  of  the  water  taken  in  than  do  other  species. 
The  two  extremes  in  this  respect  are  desert  plants,  such 
as  cacti,  sage-brush,  and  euphorbias,  and  water-loving 
plants,  such  as  water-lilies,  ferns,  touch-me-not  or  jewel- 
weed,  and  cucurbits  like  pumpkin  and  squash.  A  con- 
venient method  of  measuring  these  differences  is  to  com- 
pare the  weight  of  water  absorbed  with  the  weight  of 
dry  matter  produced.  This  ratio  is  known  as  the  relative 
water  requirement  of  the  plant.  Thus,  if  a  given  plant, 
during  its  growth,  has  taken  in  loo  pounds  of  water,  and 
the  soHd  matter  produced,  when  dried  out  to  a  constant 
weight  in  a  drying  oven,  weighs  2  pounds,  the  relative 
water  requirement  is  (100  :  2)  50. 

44.  Government  Experiments. — In  experiments  con- 
ducted for  the  United  States  Department  of  Agriculture, 
for  the  purpose  of  ascertaining  the  relative  water  require- 
ments of  various  plants,  it  was  found  that  the  weight  of 
water  taken  in  by  hubbard  squash  plants  amounted  in 
some  cases  to  over  6,000  times  the  weight  of  the  fruit, 
and  to  over  900  times  the  weight  of  the  total  dry  substance, 
not  including  the  roots.  Other  ratios,  in  round  numbers, 
were  ascertained,  as  follows: 


Table  I. — Water  Requirements  of  Plants 


Plant 

Water  requirement  based  on 

Grain 

Dry  matter 

Rye      

1,800 

496 

Rice.                              

744 

Flax 

Cucumber.           

2,800 
1,600 
6,400 

900 
891 

Sunflower                     

705 

Purslane                                         

300 



ABSORPTION   OF   WATER  49 

45.  Importance  of  Root-hairs. — In  Chapter  II  attention 
was  called  to  the  root-hairs.  Their  chief  function  is  the 
absorption  of  water  and  dissolved  substances  from  the 
soil.  This  may  be  demonstrated  by  a  very  simple 
experiment.  If  a  young  seedling  of  bean,  corn,  or  any 
other  plant  growing  in  soil,  is  pulled  up,  care  being  taken 
not  to  loosen  the  dirt  from  the  roots,  then  properly 
transplanted  in  another  place,  and  well  watered,  it 
will  continue  to  grow,  having  suffered  no  apparent  injury. 
If  another  seedling,  of  about  the  same  size  and  vigor  is 
pulled  up,  and  the  soil  removed  from  the  roots  by  means 
of  the  fingers,  some  of  the  very  delicate  root-hairs  will 
be  torn  off  with  the  soil,  and  many  others  will  dry  up  owing 
to  too  prolonged  exposure  to  the  air.  If  this  seedhng 
is  then  transplanted,  it  will  recover  with  difficulty,  or 
it  will  wilt  and  die,  even  though  well  watered,  showing  in 
a  very  clear  manner  the  inability  of  a  plant  to  take  in 
water  when  deprived  of  its  root-hairs. 

46.  Location  of  Root-hairs. — Root-hairs  for  study  may 
be  easily  secured  by  germinating  seeds  in  a  moist  chamber, 
formed  by  inverting  a  flower  pot  over  a  saucer  of  water. 
Small  seeds,  such  as  flax  or  white  mustard,  will  readily 
adhere  to  the  moistened  inner  surface  of  the  inverted 
flower  pot.  Within  24  to  36  hours  the  roots  will  have 
developed  to  a  length  of  several  millimeters,  and  the 
root-hairs  will  appear  as  a  delicate  white  "fuzz,"  near 
the  end  of  the  root,  but  not  extending  clear  to  the  tip 
(Fig.  36).  On  older  roots  it  may  be  seen  that  the  root- 
hairs  are  confined  to  a  relatively  short  zone,  only  a 
few  milhmeters  long.  The  hairs  nearer  the  root-tip  are 
shorter  than  those  further  back,  indicating  that  they  are 
younger. 


50  THE  VEGETATIVE    FUNCTIONS    OF    PLANTS 

The  tip  of  the  root  is  covered  by  a  root-cap  (Fig.  37), 
composed  of  cells  that  serve  to  protect  the  delicate  tip  as 
it   grows   through   the   soil.     In   some   cases,   as   in   the 


Fig.  36. — Germinating  seeds  of  white  mustard,  showing  development  of 

root-hairs. 

water-hyacinth  (Figs.  2)^  and  39),  the  root-cap  is  so  well 
developed  that  it  may  be  easily  seen  with  the  naked  eye, 
and  quite  readily  removed  and  replaced.  Root-hairs  are 
never  found  on  the  region  covered  by  the  root-cap. 


ABSORPTION    OF   WATER 


51 


47.  Structure  of  Root -hairs.— The  structure  of  root- 
hairs,  and  their  relation  to  the  root  as  a  whole,  are  illus- 
trated in  Figs.  40  and  41.  It  is  seen  at  once  that  they 
are  epidermal  cells,  elongated  at  right  angles  to  the  sur- 
face of  the  root,  forming  a  thread-likfe  sac,  closed  at  both 


k.       ^ 

^ 

-.—re-  ' 

Fig.  37. — Jack-in-the-pulpit  (Ariscstna 
triphyllum) .  Longitudinal  section 
through  a  root,  rt,  root- tip;  re,  root- 
cap.     (After  F.  L.  Pickett.) 


Fig.  38. — Roots  of  the  water- 
hyacinth  (Eichornia  crassipes 
Solms),  showing  removable 
root-caps;  b,  root-cap  removed 
from  c. 


ends.  The  typical  cell-structure  is  readily  recognized — 
cytoplasm,  nucleus,  vacuoles,  sometimes  merged  into  one 
large  vacuole,  cell-sap,  and  finally  the  cell-wall.  It  has 
recently  been  shown  that  the  cell-wall  is  composed  of  an 
inner  layer  of  cellulose  and  an  outer  layer  of  calcium 
pectate. 


52 


THE   VEGETATIVE    FUNCTIONS    OF    PLANTS 


48.  Relation  between  Root-hairs  and  Soil. — If  young, 
active  roots  are  removed  from  the  soil,  thoroughly  rinsed 
in  water,  and  examined  with  a  compound  microscope 
at  the  zone  of  root-hairs,  it  will  be  seen  that  tiny  par- 
ticles of  soil  have  adhered  so  closely  to  the  cell-wall  of 


1'  .-^m^ 

J 

U  ' 

Fig.  3q. — Water-hyacinth  (Eichornia  crassipes).  Photomicrograph  of 
a  longitudinal  section  of  a  root,  showing  the  mode  of  origin  of  lateral  roots 
{i.e.  endogenous),     a,  b,  c,  lateral  roots;  r,  t,  root-cap. 

the  hair  that  they  were  not  washed  ofif;  in  fact,  they  can- 
not be  removed  without  tearing  the  hair.  They  appear 
to  be  imbedded  in  the  cell- wall  (Fig.  42),  and  are  firmly 
held  by  pectin  mucilage,  resulting  from  a  transformation 
of  the  outer  membrane  of  calcium  pectate.  Since  pectin 
is  a  water-loving  colloid  its  importance  here  is  recognized 
at  once,  in  connection  with  the  absorption  of  liquids  by 
the  root-hairs. 


ABSORPTION   OF   WATER 


53 


Fig.      40. — Diagram  Fig.  41. — Root-hairs  from  the  root  of  a  mus- 

sho wing  relation  of  root-  tard  seedling,     a,  In  state  of   turgor;   h,   be- 

hairs  to  adjacent  cells  of  ginning  of  plasmolysis  after  immersion  in  weak 

the  root,     h,  the  oldest  salt-solution;  c,  later  stage  of  plasmolysis;  «, 

of  the  four  hairs  shown,  nucleus, 
and   furthest  from  the 
root-tip. 


Fig.  42. — Root-hairs,  with  soil-particles  adhering.     (After  Sachs.) 


54 


THE   VEGETATIVE    FUNCTIONS    OF    PLANTS 


49.  Relation  between  the  Water  and  the  Soil. — Fig. 

43  is  a  diagram,  showing  on  a  greatly  enlarged  scale, 
how  the  root-hairs  lie  in  the  soil,  and  the  condition  of  the 
soil  most  desirable  for  the  well-being  of  the  plant.  It 
is  seen  from  the  figure  that  the  soil  is  not  compact,  but 
open  or  porous,  the  soil  particles  being  separated  by 
spaces  as  large  or  larger  than  themselves.  Under  con- 
ditions   most    favorable    for    the    plant,    the    spaces    are 


I 


Fig.  43. — Diagram  to  illustrate  a  root-hair  (A)  in  the  soil,  and  its 
relation  to  the  soil-particles,  the  capillary  film  of  water  (w),  and  the  air 
spaces  (a);  e,  epidermal  cell  of  the  root,  of  which  the  root-hair  is  an  out- 
growth, or  branch.     (After  Sachs.) 

filled  with  air,  while  each  particle  of  soil  is  surrounded  by  a 
thin  film  of  water.  This  is  the  water  that  supplies  the 
plant  through  the  root-hairs.  As  fast  as  removed  it  is 
replenished  by  the  capillary  action  of  the  soil.  Roots 
will  continue  to  remove  the  capillary  water  from  the  soil 
until  a  point  is  reached  where  the  attraction  of  the  soil- 
particles  for  the  water  exceeds  the  absorbing  power  of  the 
root-hairs;  then  the  plant  will  wilt  unless  more  water  is 
added.     Plants  cannot  absorb  all  the  water  from  the  soil. 


ABSORPTION   OF  WATER  55 

50.  Advantage  of  the  Air  Spaces.— Living  roots,  like 
everything  else  alive,  need  fresh  air  for  respiration.  If 
the  spaces  between  the  soil  particles  were  filled  with  water, 
the  air  would  be  driven  out,  and  the  root-hairs  could  not 
respire.  They  would  soon  cease  to  function  at  all,  and 
ultimately  the  whole  plant  would  die.  Thus  it  is  seen 
that  plants  may  have  too  much  water,  as  well  as  too 
little.  Farmers'  crops  (notably  corn)  often  suffer  from 
this  cause,  as  well  as  from  drought.  When  the  soil 
contains  too  much  water  the  leaves  will  commonly  turn 
yellow  and  die.  In  order  to  understand  how  the  soil- 
water  passes  into  root-hairs,  it  is  necessary  to  understand 
the  physical  actions  of  diffusion  and  osmosis. 

51.  Diffusion  of  Gases.— If  a  bottle  of  musk,  or  other 
perfume,  is  opened  in  one  corner  of  a  room,  free  from  all 
air  currents,  a  person  standing  some  distance  away  could, 
in  time,  detect  the  odor.  Now  the  only  way  we  can  smell 
a  substance  is  to  have  one  or  more  particles  of  that 
substance,  in  gaseous  form,  touch  the  olfactory  surfaces 
of  the  nose.  Therefore,  in  the  case  of  the  musk,  tiny 
invisible  particles  must  have  left  the  surface  of  the  sub- 
stance, passed  up  through  the  neck  of  the  bottle,  out  into 
the  room,  and  travelled  (though  without  any  air  currents) 
to  the  person  detecting  the  odor.  This  illustrates  diffusion 
of  gases. 

52.  Diffusion  of  Liquids. — If  a  small  quantity  of  sugar 
could  be  deposited,  through  a  glass  tube,  at  the  bottom 
of  a  tall  tumbler  filled  with  water,  the  sugar  would  first 
dissolve,  and  the  water  near  the  bottom  would  become 
sweet.  If  we  carefully  avoided  stirring  the  water,  and  if 
all  currents  in  the  liquid  were  avoided,  nevertheless, 
within  a  short  time  the  water  on  the  surface  would  taste 


56  THE    VEGI'/iATIVE    E UNCTIONS    OF    PLANTS 

sweet,  showing  that  some  of  the  dissolved  sugar  had 
passed,  by  its  own  motion,  up  through  the  mass  of  the 
water  to  the  top.  This  simple  experiment  illustrates  difu- 
sion  of  liquids.  The  dissolved  sugar  behaves  in  a  manner 
quite  similar  to  that  of  the  gaseous  "odor"  of  the  musk. 

53.  Osmosis. — If,  now,  the  denser  sugar  solution  in  the 
bottom  of  the  tumbler  were  separated  from  the  less 
dense  water  above  by  a  porous  membrane,  such  as  a  piece 
of  bladder  or  parchment,  the  diffusion  would  take  place 
through  the  porous  membrane,  and  the  water  above  would 
soon  become  sweet,  as  in  the  previous  case.  In  other 
words,  it  is  possible  to  have  diffusion  through  a  membrane. 
Di fusion  through  a  membrane  is  osmosis. 

The  conditions  realized  in  the  experiment  described 
above  are  a  denser  liquid  (in  the  bottom  of  the  tumbler) , 
separated  from  a  less  dense  liquid  (at  the  top  of  the 
tumbler)  by  a  porous  membrane.  Moreover,  not  only 
would  the  sugar  solution  pass  up  through  the  membrane, 
but  the  water  above  would  pass  in  the  opposite  direction, 
and  more  rapidly  than  the  sugar  solution.  This  would 
continue  until  the  solution  was  of  the  same  density 
(equal  amounts  of  sugar  in  equal  amounts  of  water) 
on  both  sides  of  the  membrane.  Thus  the  action  of 
osmosis  may  be  stated  as  follows: 

When  two  fluids  {liquids  or  gases)  of  different  densities 
are  separated  by  a  porous  membrane,  di  fusion  through  the 
membrane  will  take  place  until  equilibrium  results.  The 
difusion  will  be  more  rapid  from  the  less  dense  to  the  more 
dense  fluid. 

Or,  again,  osmosis  may  be  defined  as  the  interchange  oj 
two  fluids  of  diferent  densities  when  separated  by  a  porous 
membrane. 


ABSORPTION   OF   WATER 


57 


54.  Demonstration  of  Osmosis. — The  contents  of  a 
hen's  egg  are  enclosed  by  a  porous  membrane  closely  ap- 
pressed  to  the  inside  of  the  shell,  except  at  the  large  end 
of  the  egg.  At  this  end  the  shell,  as  may  readily  be  seen, 
is  more  porous  than  elsewhere,  so  that  air  rea,dily  enters, 
pressing  the  membrane  in,  and  forming  an  air-chamber 
between  it  and  the  shell.  With  a  sharp-pointed  knife 
the  shell  may  here  be  punctured,  and  with  the  aid  of  small 
scissors,  removed  so  as  to  make  an  opening  from  J^  to 
%  inch  in  diameter.     The  greatest  care  must  be  taken  not 


Fig.  44. — Experiment  with  an  egg  to  demonstrate  osmosis.  A,  at  the 
beginning;  B,  about  one  hour  later;  w,  water  surface;  s,  support;  m,  egg- 
membrane. 


to  puncture   the   membrane,   which   at  this   region,   lies 
concave  (Fig.  44). 

The  contents  of  the  egg  are  the  yolk,  the  ''white" 
(albumen) ,  and  an  aqueous  solution  of  various  salts,  which 
permeates  the  yolk  and  the  "white."  If,  now,  the  egg 
is  placed  upright  in  a  glass  of  water,  so  as  to  be  completely 
covered  by  the  water,  we  shall  have  realized  the  condi- 
tions for  osmosis,  the  two  liquids  being  the  water  outside 
and  the  aqueous  solution  inside  the  egg.  Within  a  short 
time  the  less  dense  water  will  have  passed  through  the 
membrane  so  much  more  rapidly  than  the  dissolved  salts 


58  THE   VEGETATIVE   FUNCTIONS    OF   PLANTS 

pass  out,  that  the  membrane  will  begin  to  bulge,  becom- 
ing convex  and  in  a  state  of  tension.  This  condition  is 
known  as  turgor,  and  the  membrane  is  said  to  be  turgid. 
The  turgor  is  the  result  of  the  osmotic  pressure  of  the  solu- 
tion within.  If  osmosis  is  allowed  to  continue  after  this 
condition  is  realized,  the  osmotic  pressure  will  rupture 
the  membrane,  and  allow  the  contents  of  the  egg  to  escape. 
It  is  partly  the  osmotic  pressure  of  the  substances  in  solu- 
tion in  the  cell-sap  that  keeps  the  lining  layer  of  cytoplasm 
closely  appressed  to  the  cell-wall  of  the  root-hair,  and  other 
cells. 

55.  Application  to  Root-hairs. — The  application  of  the 
experiment  with  the  egg  to  root-hairs  in  the  soil  is  obvious. 
The  cell-sap  in  the  vacuole  is  the  denser  liquid,  the  soil- 
water  (a  very  weak  solution  of  various  substances  dis- 
solved from  the  soil)  is  the  less  dense,  while  the  two 
limiting  membranes  of  the  layer  of  cytoplasm  lining  the 
inner  surface  of  the  cell- wall  constitute  two  porous  osmotic 
membranes.  Under  normal  conditions  an  interchange 
(osmosis)  begins  between  the  cell-sap  and  the  soil-solu- 
tion. The  latter  soaks^  through  the  thin  cell-wall,  passes 
by  osmosis  through  the  outer  limiting  membrane,  diffuses 
through  the  lining  layer  of  cytoplasm  until  it  reaches  the 
inner  limiting  membrane,  through  which  it  passes  by 
osmosis  into  the  vacuole,  and  becomes  a  part  of  the  cell- 
sap.  This  process  is  sometimes  referred  to  as  endosmosis 
(osmosis  from  without  in).  In  reverse  order,  minute 
traces  of  various  substances  pass  out,  by  exosmosis. 
Doubtless  the  dissolved  substances  that  enter  the  cell 
from   without   are   in   part   altered,  chemically,  as  they 

^  It  is  not  necessary,  here,  to  attempt  to  explain  this  process  of  soaking 
{imbibition),  in  the  terms  of  physics. 


ABSORPTION  OF   WATER 


59 


diffuse  through  the  cytoplasm  between  the  two  limiting 
or  surface  membranes. 

66.  Plasmolysis.— If  the  closed  sac,  formed  by  the 
porous  membrane,  contains  the  less  dense,  instead  of  the 
more  dense  liquid,  then  the  reverse  of  turgor  will  take 
place,  and  the  sac  will  collapse.  This  may  be  easily 
demonstrated  under  the  microscope  by  irrigating  root- 
hairs,  or  other  plant  cells,  with  solutions  more  dense  than 
the  cell-sap.  For  example,  the  root-hairs  shown  in 
Fig.  41,  mounted  in  water  on  a  glass  slide,  under  a  cover- 
glass,  were  found,  by  microscopic  examination,  to  be 
turgid.  Then  they  were  irrigated  with  a  5  per  cent, 
solution  of  common  table  salt.  This  solution  is  denser 
than  the  cell-sap  of  the  root-hairs,  so  that  exosmosis  was 
more  rapid  than  endosmosis,  and  cell-sap  was  withdrawn 
from  the  vacuoles  faster  than  liquid  entered  from  without. 
The  salt-solution  having  soaked  through  the  cell-wall, 
passed  with  difficulty  through  the  limiting  membrane  of 
the  cytoplasm,  and  thus  began  to  exert  an  osmotic  pressure 
from  without,  which  loosened  the  protoplasm  {plasmoly- 
sis^), and  caused  it  to  collapse.  .When  this  results,  the 
cell  is  then  said  to  be  plasmolyzed. 

67.  Importance  of  Osmosis. — No  physical  phenomenon 
is  more  important  than  osmosis.  Upon  it  depends  the 
life  and  death  of  every  living  thing.  By  it,  not  only  do 
plants  take  in  necessary  substances  from  the  soil,  but  all 
the  food  assimilated  by  man  and  the  lower  animals  passes 
into  their  cells.  It  has  been  demonstrated  that  the  mainte- 
nance of  turgidity  is  necessary  in  order  that  cells  may  con- 
tinue to  perform  their  normal  functions.  In  a  state  of 
plasmolysis  they  cannot  do  so.     This  is  illustrated  in  a 

^  From  the  Greek,  plasma  -{-  luein,  to  loose,  or  set  free. 


6o  THE   VEGETATIVE    FUNCTIONS    OF    PLANTS 

simple  manner  by  the  well-known  fact  that,  if  a  quantity 
of  salt  is  placed  on  the  soil  around  a  plant,  the  plant  w^ill 
soon  die.     The  reason  is  now  obvious. 

68.  Rigidity. — Attention  has  been  called  to  the  fact 
that  water  serves  as  the  vehicle  by  which  substances  in 
the  soil  are  carried  into  the  roots  and  transported  to  all 
parts  of  the  plant.  But  the  water  serves  another  use  in 
helping  to  keep  the  parts  of  the  plant  rigid,  and  thus  main- 
taining their  form.  This  service  is  accomplished  chiefly 
by  means  of  the  osmotic  pressure  which  obtains  in  every 
individual  cell.  If  every  cell  is  turgid,  tissues  as  a  whole, 
and  the  organs  of  which  they  form  a  part  will  be  rigid. 
This  may  be  easily  demonstrated  by  plasmolyzing  the 
cells  in  a  piece  of  rigid  plant  tissue,  and  then  restoring 
their  turgor. 

A  fresh  piece  of  a  beet  or  turnip,  about  2  inches  long, 
3.^  inch  wide,  and  y^  inch  thick  will  be  found  to  be  quite 
rigid,  so  that  it  cannot  be  easily  bent  without  breaking. 
If  the  piece  is  now  placed  in  a  5  per  cent,  solution  of  table 
salt  for  10  or  15  minutes  or  longer,  it  will  be  found  to  have 
lost  its  rigidity,  and  may  be  bent  nearly  double  without 
breaking.  The  salt-solution  as  we  know,  caused  the 
plasmolysis,  and  consequent  loss  of  turgor  of  every  cell, 
and  so  the  entire  tissue  became  flabby. 


CHAPTER  VI 
THE  PATH  OF  LIQUIDS  IN  THE  PLANT 

59.  The  Problem  Stated. — We  have  seen  that  plants 
are  continually  losing  water  by  transpiration,  chiefly  from 
the  leaves,  and  making  good  the  loss  by  absorption  through 
the  roots.  The  question  now  arises  as  to  how  the  water 
passes  through  the  stem  to  the  leaves.  Does  it  pass 
through  the  entire  tissue  of  the  stem,  or  is  it  confined  to 
definite  regions  or  channels? 

60.  Demonstration  of  Channels. — It  will  be  easy  to 
solve  our  problem  experimentally  by  placing  various 
stems  or  branches  in  hquid  containing  some  coloring 
substance  which  will  stain  the  tissues  through  which  it 
passes.  Common  red  ink  may  be  used  for  this.  Into 
water,  colored  with  red  ink,  may  be  placed  young  seedling 
corn  plants,  stalks  of  celery,  seedlings  of  castor-oil  plants, 
leaves  of  plantain  and  lily,  parsnips  with  a  portion  of 
the  small  end  cut  aw^ay,  or  any  other  available  material. 
After  the  stems  have  been  allowed  to  stand  in  the  ink 
solution  over  night,  they  should  be  thoroughly  rinsed,  to 
remove  the  stain  from  the  surface^  and  then  examined 
by  cutting  off  a  small  portion  of  the  submerged  buds.  It 
will  be  clearly  seen  that  the  red  coloring  matter  is  not 
deposited  throughout  the  tissue,  but  is  confined  to 
clearly  marked  channels — the  fibro-vascular  bundles.  These 
bundles  will  be  found  to  be  distributed  differently  ac- 
cording to  the  kind  of  plant  or  its  age,  or  both.     Two 

6i 


62 


THE   VEGETATIVE   FUNCTIONS    OF   PLANTS 


general  types  of  distribution  will  be  recognized,  repre- 
sented respectively  by  the  corn  or  the  lily,  and  by  the 
castor-oil  plant  or  the  parsnip. 

61.  Internal  Structure  of  the 
Corn  Stem. — In  the  corn  stalk, 
the  fibro-vascular  bundles  are  dis- 
tributed thickly  and  irregularly 
through  the  fundamental  tissue 
(parenchyma)  of  the  stem  (Fig.  45). 
They  are  somewhat  more  numerous 
near  the  outer  rind.  A  longitudi- 
nal section  shows  the  bundles  in  side 
view,  extending  through  the  stem. 
The  corn  stalk  represents  a  type 
of  structure  {monocotyledonous) 
common  to  all  grasses^  and  closely 
related  plants,  and  often,  though 
misleadingly,  called  endogenous. 
Ingrowth,  the  new  tissue  originates 
(with  few  exceptions,  e.g.,  Yucca) 
only  at  the  tip  of  the  stem.  As  a 
rule,  growth  in  thickness  results 
only  by  the  enlargement  of  cells 
already  formed,  without  involving 
the  formation  of  new  ones. 

62.  Internal   Structure  of  the 
Castor-oil  Plant  Stem. — A  type 
of  structure  quite  different  from 
that  of  the  corn  stalk   is   illus- 
trated in  the  stem  of  the  castor-oil  plant  (Fig.  46).     Here 
the    fibro-vascular    bundles    (in    the    young    stem)    are 

^  The  Indian  corn  (maize)  belongs  to  the  family  of  Grasses  (Gramineae). 


Fig.  45. — Fibro-vascular 
bundles  in  a  corn  stalk  {Zea 
Mays). 


THE  PATH  OF  LIQUIDS  IN  THE  PLANT  63 


Fig.  46. — Diagram  showing  tissue-systems  in  young  stem  of  castor-oil 
plant  (Ricinus  cctnmunis),  as  seen  in  cross-section,  ep,  epidermis; 
cor,  cortex;  p,  pith  or  medulla;  b,  fibro-vascular  bundle;  ph,  phloem; 
ca,  cambium;  x,  xylem;  ic,  interfascicular  cambium. 


Fig.   47, — Castor-oil   plant    {Ricinus   communis).     Cross-section  of   the 
stem  of  a  plant  about  two  years  old. 


I 


64       THE  VEGETATIVE  FUNCTIONS  OF  PLANTS 


L_Jn 


Fig.  48. 


Fig.  49- 

Fig.  48. — White  lupine  {Lupiniis  albiis).  Made 
semi-transparent  by  being  placed  for  several  hours 
in  a  weak  aqueous  solution  of  potassium  hydroxide. 
After  this  treatment  the  fibro-vascular  system  may 
be  clearly  seen  (through  the  surrounding  tissues) 
by  transmitted  light,  cot,  cotyledons;  /;,  hypocotyl; 
r,  main  root;  br,  hr^,  branch  roots  of  different  ages. 
Note  the  endogenous  origin  of  the  branch  roots. 

Fig.  49. — The  system  of  leaf-veins  in  a  leaf  of  the 
common  rubber-plant  {Ficiis  elaslka).  The  veins  are 
connected  with  the  root  by  means  of  the  vascular 
bundles  extending  through  the  stem  and  roots. 


THE  PATH  OF  LIQUIDS  IN  THE  PLANT        65 

distributed  quite  regularly  in  a  circle,  surrounding  the 
i:entral  pith  {medulla).  This  central  tissue  extends  out 
between  the  fibro- vascular  bundles,  forming  the  pith-rays, 
or  medullary  rays.  The  tissue  outside  the  zone  of  bundles 
is  the  cortex. 

In  older  stems  of  this  type  the  bundles  increase  in 
number  until  a  nearly  continuous  cylinder  of  vascular 
tissue  results  (Fig.  47),  broken  only  by  numerous  thin 
medullary  rays.  In  cross-sectional  view  this  cylinder,  of 
course,  appears  as  a  circle.  Stems  having  this  arrange- 
ment of  tissues  grow  by  the  formation  of  cylinders  of  new 
tissue,  outside  of,  and  surrounding  the  older  woody  tissue. 
On  this  account  they  are  called  exogenous  (outside  growing) 
stems,  or,  preferably,  dicotyledonous  stems. 

63.  Extension  of  Vascular  Tissue  into  the  Roots  and 
Leaves. — As  shown  in  Fig.  48,  the  vascular  bundles  of 
the  stem  continue  down  into  the  roots,  branching  out  into 
the  smallest  rootlets,  and  connecting  with  the  tissue  which 
lies  next  to  the  epidermis  with  its  root-hairs.  Thus  it  is 
seen  that  there  is  an  unbroken  connection  of  vascular  tissue 
from  the  roots  to  the  leaves.  Root-hairs  and  leaf-veins 
(Figs.  49  and  50)  are  the  opposite  extremities  of  this  sys- 
tem, which  serves  for  the  conduction  of  liquids  through 
the  plant. 

64.  Structure  of  the  Fibro-vascular  Bundles.— When 
cross-sections  of  the  bundles  of  the  type  shown  in  the 
castor-oil  plant  are  examined  under  the  microscope, 
they  appear  somewhat  wedge-shaped,  with  the  smaller 
end  pointing  toward  the  center  of  the  stem  (Fig.  51). 
Three  well-defined  regions  may  be  clearly  distinguished, 
as  follows: 

I.  The  xylem,  at  the  pointed  end  of  the  bundle,  and 
s 


66 


THE   VEGETATIVE    FUNCTIONS    OF   PLANTS 


Fig.  50. — Photomicrograph  of  a  portion  of  the  system  of  veins  in  a 
leaf  of  the  rubber-plant  (Ficus  elastica).  Enlarged  about  six  times  from 
a  portion  of  Fig.  49. 


Fig.  51. — The  castor-oil  plant  {Ricinus  communis).  Portion  of  cross- 
section  of  young  stem,  co,  Cortex;  p,  pith  or  medulla;  e,  epidermis;  ph, 
phloem;  cfl,  cambium;  x,  xylem.  The  last  three  elements  compose  the 
fibro-vascular  bundle,  the  cambium  being  continuous  from  bundle  to 
bundle;  the  portion  between  the  bundles  is  called  interfascicular  cambium. 


THE  PATH  OF   LIQUIDS  IN  THE  PLANT        67 

occupying  most  of  its  area.  The  cell-walls  of  the  xylem 
are  thicker  than  those  of  the  other  cells  of  the  bundle, 
and  have  begun  to  be  transformed  into  wood,  hence  the 
name  xylem.  2.  The  phloem,  at  the  opposite,  or  blunt 
end  of  the  bundle.  The  phloem  forms  part  of  the  bark. 
3.  The  cambium,  between  the  xylem  and  phloem,  com- 
posed of  extremely  thin-walled  cells,  and  the  narrowest 
of  the  three  regions.  The  cambium  is  embryonic  tissue, 
with  its  cells  in  a  state  of  active  division.  The  new  cells 
formed  next  the  xylem  soon  become  transformed  into 
xylem-cells;  those  formed  next  the  phloem,  into  new 
phloem.  The  cambium  itself  persists  throughout  the 
life  of  the  plant.  It  is  perpetual  embryonic  tissue,  never 
becoming  entirely  transformed,  but  giving  rise  to  new 
cells  on  either  side  so  long  as  the  plant  remains  alive.  A 
strand  of  cambium  extending  between  the  bundles  {inter- 
fascicular cambium)  gives  rise  to  new  bundles,  as  well  as 
to  new  fundamental  tissue.  In  time  the  bundles  increase 
in  thickness,  and  become  so  numerous  and  close  together 
that  there  is  an  almost  continuous  cylinder  of  wood  in- 
side the  cambium,  and  a  cylinder  of  phloem  and  other 
tissues  outside  the  cambium. 

65.  Passage  of  Liquids  through  the  Stem. — The  water 
and  dissolved  mineral  substances,  taken  in  by  the  root- 
hairs,  pass  up  through  the  xylem  to  the  leaves,  while  the 
plant-food,  manufactured  in  the  leaves,  passes  down 
through  the  phloem,  and  is  distributed  to  all  living 
tissues.  The  liquids  passing  through  the  stem  are 
popularly  called  "sap.'^ 

66.  Economic  Value  of  Maple  Sap. — In  the  case  of 
the  sugar-maple,  a  very  sweet  sap  flows  in  unusually  large 
quantities   during   the   early   spring  period   of   alternate 


68  THE   VEGETATIVE    FUNCTIONS    OF    PLANTS 

thawing  and  freezing.  During  this  season,  in  many  parts 
of  the  country,  it  is  customary  to  ''tap"  the  trees,  by 
boring  a  small  hole  into  the  trunk  far  enough  to  enter  the 
wood,  and  then  insert  a  wooden  or  metal  "spiggot"  or 
spout,  through  which  the  sap  flows  out  and  is  caught  in 
pails.  It  is  then  ''boiled  down,"  either  in  the  sugar  bush 
or  in  the  house,  until  the  water  has  passed  off  in  large 
quantities  as  steam,  leaving  a  thickened  maple  syrup  in 
the  kettle  or  evaporating  pan.  If  the  boiling  is  continued 
the  syrup,  when  cooled,  becomes  maple  sugar.  The  saps 
of  the  sugar-cane  and  of  the  sugar-beet  are,  as  is  well 
known,  the  source  of  the  ordinary  sugars  of  commerce. 


CHAPTER  VII 
NUTRITION 

67.  Organic  and  Inorganic. — All  substances  belong  to 
either  one  or  the  other  of  two  classes  of  matter — organic 
or  inorganic.  Organic  substances  are,  for  the  most  part, 
those  which  compose  the  bodies  of  animals  or  plants,  past 
or  present,  or  which  have  been,  or  may  be,  formed  by  the 
life-processes  of  living  things.  The  possibility  of  synthe- 
sizing certain  organic  compounds  (hydrocarbons)  artifi- 
cially in  the  laboratory  has  broken  down  the  hard  and 
fast  distinction,  formerly  recognized,  between  organic  and 
inorganic  substances.  Bone,  flesh,  shells,  bark,  wood, 
leaves,  gums  and  resins  formed  by  plants,  coal,  sugar, 
flour,  starch,  cellulose,  plant  and  animal  juices,  and  all 
protoplasm  represent  organic  substances.  Inorganic  sub- 
stances are  those  which  have  never  been  incorporated  into 
the  bodies  of  plants  or  animals,  or  if  so,  have  since  lost  all 
evidence  of  that  fact.  Water,  salt,  iron,  oxygen,  carbon, 
glass,  sulphur,  air,  represent  inorganic  substances.  Some- 
times the  line  is  hard  to  draw.  Thus  a  piece  of  wood  or 
of  bone  converted  into  charcoal  may,  if  carefully  handled, 
retain  unmistakable  traces  of  having  formed  a  part  of 
the  body  of  an  animal  or  a  plant,  but  if  the  piece  is  ground 
in  a  mortar,  a  fine  powder  may  result,  that  has  lost  all 
trace  of  its  organic  origin.  Ordinarily,  however,  the 
two  kinds  of  matter  are  easily  distinguished,  either  by 
their  structure  or  their  known  origin.  So  clearly  distinct 
and  unlike  are  they,  that  one  entire  branch  of  the  science 

69 


70 


THE   VEGETATIVE    FUNCTIONS    OF    PLANTS 


of  chemistry,  organic  chemistry,  is  devoted  to  a  study  of 
organic  substances,  all  of  which  are  compounds  of  carbon. 
Inorganic  chemistry  is  concerned  with  all  other  substances. 
68.  Nutrition  of  Plants  and  Animals. — The  foods  of 
every  living  thing — those  substances  which,  by  digestion 
and  assimilation,  can  be  incorporated  into  the  structure 

Animals 

Non- 
chlorophyll 
bearing 
plarTte 


Chlorophyll- 
bearing 
plants 


Dieintegratlon 

by  saprophytes 

&  paraeltee 


Inorganic 

matter 

(Chemical 

elements  and 

compounds) 


Fig.  52. — The  organic  cycle 


of  the  living  body  as  a  part  of  it — are  all  organic.  In  this 
respect  animals  and  plants  are  alike;  they  both  require 
organic  substances  as  food.  Thus  it  is  evident  that  there 
must  be  a  continual  formation  of  organic  compounds  out 
of  inorganic  in  order  to  maintain  the  food  supply  of  the 
world.  As  we  shall  soon  learn,  this  is  the  great  and  funda- 
mental role  of  green  plants  in  the  world's  economy — to 
elaborate  organic  compounds  for  food  out  of  inorganic 


NUTRITION  71 

'*raw  materials/'  With  a  very  few  exceptions  among  the 
bacteria,  plants  that  are  not  green  (such  as  toad-stools, 
molds,  and  other  fungi),  and  most  animals  cannot  do  this. 
Therefore  we  are  wholly  dependent  upon  green  plants  for 
the  food  supply  of  the  world.  Animals  and  non-green 
plants  live,  either  directly  or  indirectly,  upon  green  plants. 
The  bodies  of  all  living  things  are  constantly  giving  off 
inorganic  compounds  (such,  for  example,  as  the  carbon 
dioxide  given  off  in  respiration,  and  the  water  in  trans- 
piration), and  after  death  all  bodies  are  (by  the  action  of 
bacteria)  gradually  broken  up  into  inorganic  substances. 
Thus,  we  see,  there  is  a  kind  of  perpetual  cycle,  or  cir- 
culation, from  one  realm  to  the  other,  as  indicated  in  the 
diagram  (Fig.  52). 

69.  Kinds  of  Foods. — When  we  examine  the  bodies  of 
plants  we  find  that  the  foods  elaborated  are  stored  in 
various  organs  or  tissues.  These  foods  all  belong  to  one 
of  three  classes  of  substances,  viz.,  carbohydrates^  re- 
presented by  starch,  sugars,  and  cellulose;  proteins, 
represented  by  protoplasm  itself,  the  gluten  of  wheat, 
and  other  substances;  and/a/5,  represented  by  the  various 
oils,  such,  for  example,  as  olive  and  cotton-seed  oil. 

70.  Chemical  Composition  of  These  Foods. — A  chemical 
examination  of  carbohydrates  reveals  the  fact  that  they 
are  composed  of  the  elements,  carbon,  hydrogen,  and 
oxygen,  the  two  latter  occurring  in  the  same  proportion 
as  in  water,  namely,  two  parts  of  hydrogen  to  one  of  water 
(H2O).  Fats  contain  the  same  elements,  only  in  different 
proportions,  while  proteins,  in  addition  to  carbon,  oxygen, 
and  hydrogen,  always  contain  nitrogen.  Other  sub- 
stances almost  universally  found  in  plants  are  calcium, 
potassium,   magnesium,   phosphorus,   sulphur,   iron,   and 


72  THE   VEGETATIVE   FUNCTIONS    OF   PLANTS 

chlorine.  Calcium  is  not  necessary  for  all  fungi  (e.^., 
not  for  Aspergillus  niger),  nor  for  all  algae.  Three  main 
problems  now  confront  us: 

1.  What  is  the  source  of  these  food  elements? 

2.  Where,  in  the  plant,  are  they  elaborated  into  plant 
food? 

3.  How  is  the  process  accomplished? 

71.  Source  of  the  Food  Elements. — Since  most  plants 
are  fixed  for  life  to  a  certain  spot  they  must  obtain  their 
food  elements  from  their  immediate  surroundings;  they 
cannot,  like  animals,  go  in  search  of  them.  The  carbon 
and  oxygen  are  obtained  chiefly  from  the  air,  where  the 
carbon  appears,  in  combination  with  oxygen,  as  carbon 
dioxide.  Free  oxygen  (as  well  as  in  combination  with 
carbon)  is  also  obtained  from  the  air.  About  four-fifths 
of  the  air  is  nitrogen,  and  one  might  naturally  infer 
that  the  plant  could  obtain  an  abundant  supply  from 
that  source.  We  shall  learn  later,  however,  that  most 
plants  cannot  utilize  the  free  nitrogen  of  the  air,  but 
must  have  it  supplied  in  chemical  combination  with 
other  elements,  in  the  form  of  nitrates.  These  are 
obtained  from  the  soil.  The  calcium,  potassium,  mag- 
nesium, phosphorus,  sulphur,  and  iron  are  also  obtained 
from  the  soil  in  the  form  of  soluble  salts,  such  as  phos- 
phates, nitrates,  sulphates,  and  carbonates.  The  hy- 
drogen is  obtained  chiefly  from  water  (H2O). 

72.  Seat  of  Elaboration  of  Carbohydrates. — Careful 
and  thorough  studies  have  established,  beyond  all  doubt, 
the  fact  that  the  inorganic  food  elements  are  combined 
to  form  carbohydrates  only  in  the  green  cells  of  plants, 
either  in  leaves,  stems,  or  other  parts.  This  is  one  of  the 
most    fundamental    facts   in    all   science.     Among  those 


NUTRITION 


73 


^'""'thp'^ffr  ^"Sen-Housz  (1730-1799).     He  was  the  first  to  recognize 
the  difiference  between  photosynthesis  and  respiration  in  plants 


Fig.  54.-Joseph  Priestley    (1733-1804).     He    discovered   oxygen,   and 
made  pioneer  studies  of  the  function  of  chlorophyll. 


74  THE   VEGETATIVE    FUNCTIONS    OF    PLANTS 

entitled  to  credit  for  this  discovery  may  be  mentioned 
Jan  Ingen-Housz  (i 730-1 799),  a  Dutch  physician,  and 
Jean  Senebier  (i 742-1 809)  and  Nicolas  Theodore  de 
Saussure  (1767-1845),  two  Frenchmen.  Joseph  Priestley 
an    Englishman,    also    contributed    to    this   work,  both 


Fig.  55. — Leaves  of  the  tulip  tree  {Liriodendron  tuUpifera).  At  left, 
from  a  large  mature  tree;  at  right,  from  a  young  sapling.  Average  sized 
leaves  were  chosen  from  each  tree.     Greatly  reduced. 

directly  and  indirectly,  by  his  discovery,  in  1774,  of  the 
gas  oxygen,  and  his  experiments  on  the  purification  of  the 
air  by  green  plants. 

73.  The  Significance  of  Leaves. — While  the  formation 
of  carbohydrate  food  may  take  place  in  any  cell  con- 


NUTRITION  75 

taining  the  green  substance,  chlorophyll  (leaf -green),  the 
chief  organs  for  this  work  are  the  leaves.  This  explains 
many  facts  about  leaves — e.g.,  why  they  are  green,  why 
they  are  thin  and  usually  broad,  why  they  are  often 
much  larger  in  young,  rapidly  growing  plants  that  need 
much  nourishment,  than  in  mature  plants  (Fig.  55), 
why  they  occur  at  or  very  near  the  tips  of  the  branches, 
where  they  are  well  exposed  to  light  (Figs.  56  and  57). 
There  is  no  more  important  fact  in  botany,  nor  indeed 
in  all  natural  science,  than  that  all  the  food  of  the  world 
is  primarily  manufactured  in  the  chlorophyll-containing  cells 
of  plants. 

74.  Importance  of  Sunlight. — Plants  and  plant  parts 
grown  in  the  dark  are,  with  rare  exceptions,  never  green. 
This  means  that  sunlight  is  necessary  in  order  to  make 
chlorophyll.  But  green  plants  cannot  elaborate  food  in 
the  dark.  This  means  that  sunlight  is  necessary,  not 
alone  for  the  formation  of  chlorophyll,  but  for  food  mak- 
ing as  well.  Non-green  tissues,  even  in  sunlight,  cannot 
manufacture  food;  for  this  process  both  chlorophyll  and 
sunlight  are  necessary.  The  green  cell  has  often  been 
likened  to  a  factory;  the  chlorophyll  is  the  machinery, 
the  sunlight  is  the  energy,  while  the  product  of  the  factory 
is  the  manufactured  food. 

75.  Details  of  the  Process. — The  manufacture  of  carbo- 
hydrates involves  three  essential  steps: 

1.  Taking  in  the  raw  materials  (water  and  carbon 
dioxide) . 

2.  Recombining  these  parts  into  carbohydrates. 

3.  Giving  off  the  waste  material  (chiefly  oxygen). 
Taking  in  the  Raw  Materials. — We  have  seen  in  Chapter 

IV  that  the  air  spaces  between  the  green  cells  of  a  leaf 


76  THE   VEGETATIVE    FUNCTIONS    OF   PLANTS 

are  in  direct  connection  with  the  outside  air  through  the 
stomata.  In  the  presence  of  sunlight  the  formation  of 
carbohydrates  begins  in  the  green  cells.  This  uses  up 
the  carbon  dioxide  in  the  cells,  and  the  supply  is  renewed 
from  the  intercellular  spaces.  The  gas  passes  through 
the  cell-walls  and  the  layer  of  protoplasm  in  solution  in 


Fig.  56. — A  tree  (hornbeam),  seen  from  ''outside,"  showing  the 
dense  foliage  at  or  near  the  tips  of  the  branches.  The  same  tree  as  in 
Fig.  57- 

water.  This  results  in  reducing  the  amount  (and  thus 
the  pressure)  of  this  gas  in  the  intercellular  spaces,  and  as 
a  result  more  carbon  dioxide  passes  by  diffusion  through 
the  stomata  to  the  intercellular  spaces.  Thus,  as  fast 
as  the  gas  is  used  the  supply  is  renewed  from  without. 

76.  Photosynthesis. — Within  the  cell,  the  carbon  di- 
oxide and  water  (or  simple  combinations  of  these)  are 
finally,  hy  a  series  of  steps,  recombined  by  the  chlorophyll 


J 


NUTRITION 


77 


in  the  presence  of  sunlight,  into  a  carbohydrate — probably 
some  form  of  sugar.  //  is  this  series  of  steps  that  is  called 
photosynthesis.'^  Not  all  of  the  oxygen  contained  in  the 
water  (H2O)  and  carbon  dioxide  (CO2)  used,  enters  into 
the  composition  of  the  carbohydrate.  The  unused  por- 
tion is  set  free,  and  is  either  utilized  in  other  processes, 


Fig.  57. — A  tree  (hornbeam),  seen  from  among  the  branches  (as  the 
squirrel  sees  it),  showing  absence  of  leaves  except  at  or  near  the  tips  of  the 
branches.     The  same  tree  as  in  Fig.  56. 

or  diffuses  out  through  the  stomata  to  the  surrounding 
air.  Thus,  the  taking  in  of  carbon  dioxide  and  the 
giving  off  of  oxygen  are  outward  indications  that  photo- 
synthesis is  going  on  inside  the  green  cells. 

77.  Starch -making. — The  sugar  made  by  photosynthe- 
sis is  soluble  in  the  cell-sap,  and  if  it  were  not  removed 

^  The  word  means  combining  (synthesis)  in  the  presence  of,  or  by  means 
of,  light  (photos). 


78 


THE   VEGETATIVE    FUNCTIONS    OF   PLANTS 


from  the  solution,  at  least  as  fast  as  it  was  made,  it  would 
accumulate  and  thus  interfere  with  the  manufacture  of 
more  sugar.  Some  of  it  is  removed  at  once,  either  by 
nourishing  the  protoplasm  of  the  cell  where  it  was  made, 
or  by  being  translocated  to  other  parts  of  the  plant.  But 
some  of  the   sugar  is  removed  from  solution  by  being 


Fig.  58. — Cell  of  Pellionia  Daveauana,  showing  starch-grains.  The 
black,  crescent-shaped  body  on  the  end  of  each  grain  is  the  amyloplast. 
Greatly  enlarged.     (Cf.  Figs.  8  and  59.) 


converted  into  starch,  a  substance  not  soluble  in  water. 
Thus  the  accumulation  of  starch  in  a  leaf  or  other  green 
tissue  indicates  that  photosynthesis  is  in  progress,  and 
that  the  resulting  carbohydrate  is  or  has  been  formed 
faster  than  translocated.  The  conversion  of  sugar  to 
starch  is  accomplished  by  certain  plastids  in  the  cell 
(Figs.  8  and  58). 


NUTRITION  79 

78.  Enzymes. — For  a  long  time  it  has  been  known  that 
during  photosynthesis  plants  take  in  carbon  dioxide  and 
water  and  give  off  oxygen,  but  the  intermediate  steps 
have  never  been  clearly  understood.  The  appearance  of 
starch  in  green  tissues  is  an  evidence  that  photosynthesis 
has  taken  place,  but  it  was  early  recognized  that  starch 
was  not  the  first  organic  substance  to  be  formed.  It  is 
now  known  that  some  of  the  various  steps  in  the  process 
are  accomplished  by  means  of  certain  substances  called 
enzymes,  formed  in  every  cell.  Enzymes  have  the  re- 
markable power  of  transforming  other  substances,  with- 
out being  thereby  used  up  or  permanently  changed  them- 
selves. They  belong  to  the  class  of  substances  known  as 
ferments  J  but  their  real  nature  and  mode  of  action  are  not 
well  understood.  Each  enzyme  is  commonly  named  from 
the  particular  substance  in  the  transformation  of  which 
it  takes  part,  and  this  name  usually  ends  with  the  termi- 
nation, -ase.  Thus  we  have  oxidase,  which  acts  upon 
substances  to  oxidize  them,  maltase,  which  acts  upon  mal- 
tose (a  form  of  sugar),  protease,  which  acts  upon  protein, 
and  so  on. 

79.  The  Steps  in  Starch-formation. — Careful  experi- 
ments have  suggested  that  the  first  step  in  the  formation 
of  starch  may  be  the  interaction  of  water  and  carbon 
dioxide,  under  the  agency  of  an  oxidase,  resulting  eventu- 
ally in  formaldehyde  (CH2O).  The  subsequent  steps 
may  be  something  as  follows : 

2.  Condensation  of  the  formaldehyde  molecules  into 
a  simple  sugar,  dextrose  (C6H12O6),  by  aldehydase. 

3.  Transformation  of  dextrose  into  a  more  complex 
sugar,  maltose,  by  maltase. 

4.  The  changing  of  maltose  into  dextrine  by  dextrinase. 


So  THE   VEGETATIVE    FUNCTIONS    OF    PLANTS 

5.  The  conversion  of  dextrin  into  soluble  starch 
(amylum)  by  amylase. 

6.  The  conversion  of  soluble  starch  into  insoluble  starch 
by  yet  another  ferment  or  enzyme,  coagulase. 

Other  analyses  of  the  process  of  photosynthesis  have 
been  suggested,  and  it  is  possible  that  the  one  outlined 
above  may  become  more  or  less  modified  in  the  light  of 
future  experiments. 

80.  Storage  of  Food. — As  stated  above,  some  of  the 
food  elaborated  in  the  leaves  is  transferred  to  various 


Fig.  59. — Starch  grains  from  a  potato  tuber.     (After  Duncan  J.  Reid.) 

other  parts  of  the  plant.  Some  of  it  is  used  immediately 
to  nourish  these  parts,  but  often  this  food  accumulates 
faster  than  needed.  This  is  what  always  occurs  in  certain 
parts  of  the  plant,  such,  for  example,  as  the  tubers  (under- 
ground stems)  of  potatoes  (Figs.  59  and  60),  the  roots  of 
turnips,  the  seeds  of  beans,  peanuts,  and  other  plants, 
the  fruits  of  all  plants,  the  leaf-base  of  the  onion  (form- 
ing the  scaly  bulbs  or  "onions");  and   all   buds  when 


NUTRITION 


8l 


they  are  forming.  In  these  storage  organs  the  soluble 
sugar  is  generally  removed  from  solution  by  being  con- 
verted by  starch-forming  leucoplasts  (amyloplasls)  into 
starch,  1  and  thus  the  storage  organs  finally  become  gorged 
with  an  excess  of  food.  It  is  on  this  account  that  they  are 
valuable  as  food  for  man. 


Fig.  6o. — Young  potato  tuber,  developed  (in  light)  as  a  branch  of  a 
sprout  of  an  old  seed-tuber.  Part  of  the  food  elaborated  and  digested  in 
the  leaves  of  the  parent  plant  was  translocated  down  the  leaf-stalk  and 
stem,  and  stored  in  the  older  tuber,  part  of  which  is  shown  in  the  figure. 
After  this  piece  was  cut  off,  the  stored  food  began  to  be  digested  and  trans- 
located to  the  developing  "eye"  or  bud,  accompanied  by  the  development 
of  the  latter  into  the  young  tuber.  Ordinarily  such  changes,  in  the  potato, 
occur  only  underground  and  in  the  dark. 

81.  The  Need  and  Source  of  Nitrogen.— Protein  foods 

differ  from  carbohydrates  and  fats  by  containing  nitrogen 

which  the  latter  lack.     Notwithstanding  the  fact  that 

four-fifths  of  the  atmosphere  is  nitrogen,  green  plants  are 

unable  to  use  this  free  nitrogen  until  it  has  been  chemically 

combined  with  other  substances,  such,  for  example,  as 

1  In  the  case  of  the  onion,  for  example,  the  sugar  is  not  converted  into 
starch. 
6 


82 


THE  VEGETATIVE  FUNCTIONS  OF  PLANTS 


potassium,  calcium,  magnesium,  ammonium,  and  others, 
forming  potassium  nitrate  (KNO3),  calcium  nitrate 
(Ca(N03)2),  ammonium  nitrate  (NH4)N03,  and  so  forth. 
Nitrates  and  ammonia  may  result  from  the  disintegration 
of  plant  and  animal  bodies,  but  the  supply  must  con- 
tinually be  renewed  by  drawing  upon  the  free  nitrogen 


Fig.  61. — Root-tubercles  on  yellow  sweet  clover   {Mdilotus  officinalis), 
caused  by  and  containing  Psciidomonas  radicicola. 


of  the  air.  The  ability  to  ''fix"  atmospheric  nitrogen, 
that  is,  to  form  nitrogen  compounds  with  the  free  nitrogen 
of  the  air,  is  possessed  by  several  kinds  of  bacteria,  includ- 
ing species  of  Azotobacter,  Clostridium,  and  Pseiidomonas. 
There  is  also  evidence  that  certain  lower  fungi  possess 


NUTRITION  83 

this  function.  Pseudomonas  radicicola,  lives,  as  its  name 
implies,  in  roots,  and  chiefly  in  those  of  leguminous  plants, 
such  as  clover,  lupine,  locust,  peas,  beans,  alfalfa,  and 
their  near  relatives.  The  presence  of  the  bacterium 
causes  Httle  swellings  or  nodules  on  the  roots,  commonly 
known  as  leguminous  tubercles  (Figs.  61  and  228). 

Nitrification,  or  the  oxidation  of  ammonium  compounds 
into  nitrates,  was  at  first  thought  to  be  simply  a  chemical 
process,  but  early  in  the  nineteenth  century  it  was  found 
to  depend  upon  hving  organisms.  Numerous  bacteria, 
molds  and  other  fungi  {Mucor,  Penicillium,  Botrytis) ,  and 
the  yeast  Torula  are  active  in  the  formation  of  ammonium 
salts  by  the  disintegration  of  organic  compounds.  Follow- 
ing this  process  nitrification  results.  Two  important 
nitrifying  bacteria  live  in  the  soil;  one  (Nitrosomonas) 
forms  nitrites  from  ammonia,  and  the  other  {Nitrohactcr) 
forms  nitrates  from  nitrites  (Fig.  229). 

Root-nodules,  caused  by  nitrogen-fixing  organisms,  oc- 
cur also  on  roots  of  certain  non-leguminous  plants,  includ- 
ing Elaeagnaceae  (Oleaster  family),  Myricacese  (Bayberry 
family),  Podocarpineae,  the  g&nM?>Alnus,  of  the  Betulace^ 
(Birch  family),  and  Cycadaceae  (Cycas  family).  The 
roots  of  Cycas  contain  two  kinds  of  nitrogen-fixing  organ- 
isms, Pseudomonas  radicicola  and  Azotohacter  (Fig.  241). 

82.  Value  of  Leguminous  Crops.— Because  of  the 
presence  in  their  roots  of  organisms  that  can  use  the 
free  nitrogen  of  the  air  to  form  compounds  of  nitrogen, 
leguminous  crops  are  of  inestimable  value  to  agriculture. 
In  fact,  they  are  absolutely  necessary  in  order  to  maintain 
the  fertility  of  the  soil.  When  any  leguminous  crops 
are  harvested,  the  roots  are  left  in  the  soil  with  their 
tubercles  rich  in  compounds  of  nitrogen,  and  the  com- 


84  THE  VEGETATIVE    FUNCTIONS    OF    PLANTS 

pounds  (nitrates)  are  available  to  the  next  non-leguminous 
crop,  such  as  oats  or  corn.  This  is  one  of  the  main 
reasons  why  good  farmers  always  practice  a  rotation  of 
crops,  alternating  leguminous  with  non-leguminous  plants, 
for  thereby  the  richness  of  the  soil  in  available  nitrogen  is 
maintained.  Thus,  for  example,  a  certain  field  in  Illinois 
was  planted  to  corn  for  28  years  in  succession.  The  yield 
of  corn  for  the  last  year  was  22  bushels  per  acre.  On  a 
second  field,  planted  for  the  same  length  of  time  with 
alternate  crops  of  corn  and  oats,  the  final  yield  of  corn 
was  36  bushels;  while  a  third  field  in  which  corn,  oats,  and 
clover  were  planted  alternately,  the  final  yield  of  corn 
was  59  bushels — over  twice  the  yield  w^ithout  rotation  with 
a  legume.  This  increase  in  yield  was  due  chiefly  to  the 
enrichment  of  the  soil  in  nitrates  by  the  organisms  that 
form  tubercles  on  the  roots  of  the  clover.  If  the  entire 
clover  crop  (tops  and  all)  is  plowed  under  occasionally, 
so  as  to  serve  as  ''green  manure,"  the  results  will  be 
better  than  when  the  clover  tops  are  always  removed 
as  hay. 

83.  Manufacture  of  Proteins. — Proteins  are  formed 
de  novo  only  in  living  plant  cells,  by  combining  the  prod- 
ucts of  photosynthesis  with  the  nitrogen  supplied  in  the 
formi  of  nitrates.  Their  formation  may  be  favored  by 
light,  but  unlike  the  process  of  photosynthesis,  protein- 
formation  does  -not  require  either  light  or  chlorophyll. 
The  formation  of  proteins  occurs  in  large  measure  in 
foliage-leaves,  but  may  take  place  in  any  living  cell.  The 
manner  of  their  formation  is  not  as  well  understood  as  is 
that  of  carbohydrates. 

84.  Fats. — Fats  may  occur  in  plant  cells  in  either  liquid 
or  soKd  form.     We  are  most  familiar  with  the  liquid  fats. 


NUTRITION 


85 


such  as  the  various  oils  derived  from  plants.  Fat  occurs 
in  the  solid  form  in  the  cocoanut.  In  the  tallow-tree 
(Sapium  sehiferum)  it  is  so  abundant,  as  a  waxy  coating 
on  the  seeds,  that  it  is  used  in  eastern  Asia  for  making 
candles.  Fats  commonly  occur  in  droplets  in  the  proto- 
plasm, or  as  an  emulsion  in  the  cell-sap,  but  the  place 


1^^^^^  ' 

^^^^ifH 

f                 * 

1         *"   ^ 

^-0^     J 

^j^^^^^^^^Hl 

^^^^^^^^^^^^^^^^^^^^^iL  *' 

^^^^^^^^^H 

Fig  62  —Portion  of  a  cross-section  of  a  grain  of  Indian  corn  {Zea 
Mays)  G.E,  glandular  epithelium  of  the  scutellum  which  secretes  dias- 
tase; G,  a  simple  racemose  gland  in  the  tissue  of  the  scutellum;  D,  duct  of 
the  latter,  emptying  into  the  starchy  endosperm  surrounding  the  embryo. 

and  method  of  their  formation  have  never  been  clearly 
determined. 

85.  Digestion.— We  have  learned  above  (paragraph  42) 
that  substances  can  enter  a  plant  only  in  solution.  It  is 
also  true  that  substances,  even  when  inside  the  cell,  can- 
not be  utilized  as  food  by  the  plant  unless  they  are  in 
solution.     In  order  that  the  protoplasm  can  be  nourished, 


86 


THE    VEGETATIVE    FUNCTIONS    OF    PLANTS 


therefore,  all  insoluble  stored  foods  must  be  converted 
into  soluble  substances  and  dissolved.  The  process  of 
converting  an  insoluble  food  into  a  soluble  substance  and 
dissolving  it  is  digestion.  This  change,  like  the  various 
processes  in  photosynthesis,  is  brought  about  by  various 
enzymes,  which  have  the  power  of  converting  insoluble 


-'^'"^^1 

Q 

1      : — ~ r^ 

*     -I 

.  # 

^^1 

i  .^ 

■ 

^te^ 

^M^ 

H^k- 

,^!^fm 

^j 

^^B^ 

Fig.  63. — Venus's  flytrap  {DioncEa  muscipula).  Insects  are  captured 
by  the  rapid  closing  together  of  the  two  valves  of  the  leaf-blade.  When 
the  valves  come  together  the  rigid  marginal  teeth  interlock,  making  escape 
impossible.  The  digestive  enzyme  is  secreted  by  glands  distributed  over 
the  surface  of  the  valves.   ^  (After  John  Ellis.) 

starch,  proteins,  and  fats,  into  soluble  substances.  Starch 
is  converted  by  diastase  (one  of  the  earliest  known  enzymes) , 
proteins  by  proteases,  and  fatty  substances  (lipoids)  by 
lipase. 


NUTRITION 


87 


Digestive  enzymes  are  probably  secreted  by  every 
active  plant-cell.  In  the  corn  grain  the  corn  starch 
(endosperm),  surrounding  the  embryo,  is  digested  during 
germination  by  diastase  secreted  by  a  glandular  epithe- 


FiG.  64.— Bladderwort  {Utricularia  sp.).     Aquatic  insects  are  held  captive 
in  the  tiny  floating  bladders,  where  they  are  digested  and  absorbed. 


Fig.  65.— a  sundew  {Drosera  intermedia),  showing  the  glandular  tipped 
tentacles  on  the  leaves.  When  an  insect  alights  on  a  leaf  he  is  held  by 
the  sticky  secretion;  the  tentacles  then  bend  over  in  contact  with  his  body, 
holding  him  more  firmly,  and  pouring  out  a  protein-digesting  enzyme 
secreted  by  their  glands.  «=        &        / 


88  THE   VEGETATIVE    FUNCTIONS    OF    PLANTS 

lium  of  the  embryo.  Often  this  glandular  layer  is  in- 
vaginated,  so  as  to  form  a  true  gland  (Fig.  62). 

A  special  case  of  plant  nutrition  is  presented  by  the 
insectivorous  plants,  of  which  there  are  many  kinds. 
They  are  all  characterized  by  the  possession  of  some  de- 
vice for  capturing  insects  that  visit  them,  and  by  the 
ability  to  secrete  a  proteolytic  enzyme  capable  of  digest- 
ing the  protein  tissues  of  their  prey.  These  plants  can 
also  digest  any  meat  fed  to  them  artificially.  After 
being  digested  the  protein  food  is  absorbed  by  osmosis. 
The  Venus's  flytrap  {Dioncea  muscipula),  numerous 
species  of  bladderwort  (Utricularia),  and  several  species 
of  sundew  {Drosera)  serve  to  illustrate  the  insectivorous 
plants  (Figs.  63-65).  Although  these  plants  are  able 
to  digest  protein,  experiments  have  shown  that  a  pro- 
tein diet  is  not  necessary  either  for  their  life  or  healthy 
growth. 

86.  Assimilation. — After  food  has  been  elaborated  and 
digested  it  still  is  not  a  part  of  the  living  protoplasm,  but 
only  lies  within  the  vacuoles  of  the  protoplast.  It  is  no 
more  a  part  of  the  plant  than  is  food  in  our  hands  or 
stomachs  a  part  of  our  bodies.  One  step  more  is  nec- 
essary; the  digested  food  must  be  incorporated  into,  and 
made  part  of,  the  living  protoplasm  itself.  It  must  be 
transformed  from  lifeless  matter  into  living  matter.  As 
in  the  case  of  enzyme  action,  the  process  by  which  this 
remarkable  change  is  brought  about  is  not  understood. 
By  multitudes  of  accurate,  painstaking  experiments, 
however,  one  great  fundamental  truth  has  been  estabhshed, 
namely,  that  non-living  matter  can  be  converted  into 
living  matter  only  by  the  action  of  other  living  matter 
already   existing.     Proteins,    sugars,    and    fats   have    all 


NUTRITION  89 

been  manufactured  from  simpler  substances,  artificially 
in  the  laboratory,  without  the  aid  of  living  organisms; 
and  they  may  all  be  digested  artificially  in  a  test-tube, 
also  without  the  aid  of  living  organisms;  but,  although 
the  attempt  has  often  been  made,  no  one  has  ever  suc- 
ceeded in  artificially  producing  even  the  minutest  drop 
of  living  protoplasm.  Only  hving  protoplasm,  acting 
directly  on  non-living  matter,  can  bring  about  that 
marvellous  change.  This  fact  is  concisely  expressed  by 
the  Latin  phrase,  "Omne  vivum  e  vivo''  (All  life  from  life). 

87.  Biogenesis. — That  living  matter  is  always  descended 
from  preceding  living  matter,  and  that  it  never  arises 
spontaneously  from  the  non-living  is  the  principle  of 
biogenesis.^  Opposed  to  this  principle  is  the  principle 
of  ahio genesis,"^  which  teaches  that  living  matter  may 
originate  from  non-living  without  the  intervention  of 
other  living  matter.  This  was  formerly  quite  generally 
believed.  Men  thought,  for  example,  that  putrid  meat 
might  become  transformed  directly  into  the  maggots 
(young  flies)  so  often  found  in  it;  but  we  now  know  that 
maggots  in  decaying  meat  always  arise  from  the  eggs  of 
flies  that  have  previously  visited  the  meat  and  deposited 
their  eggs  there.  Thanks  to  the  painstaking  experiments 
and  clear  thinking  of  Redi,  Pasteur,  Tyndall,  and  others, 
belief  in  the  principle  of  biogenesis  is  now  practically 
universal  among  scientists. 

That  living  matter  could  not,  under  favorable  condi- 
tions, originate  from  non-living  matter,  or  that  it  did  not 
in  the  beginning,  or  never  does  now,  cannot,  of  course, 

^  Biogenesis,  from  the  Greek  words  hios  {pios),  life,  and  genesis  {y^vtais) 
generation. 

^  Abiogenesis.  The  prefix  a  (Greek  alpha)  deprives  the  remainder  of 
the  word  of  its  meaning,  or  reverses  the  meaning. 


90  THE   VEGETATIVE    FUNCTIONS    OF    PLANTS 

be  demonstrated;  one  can  never,  experimentally  or  other- 
wise, prove  a  universal  negative.  The  principle  of 
biogenesis  affirms  that,  in  experiments  conducted  with 
the  utmost  skill,  and  with  every  possible  precaution  to 
exclude  all  traces  of  living  matter,  no  faintest  manifesta- 
tion of  life  has  ever  been  detected ;  we  therefore  logically 
conclude  that,  however  life  may  have  been  originally 
created,  it  never  originates  now  from  non-living  matter, 
but  always  from  living  matter  only. 

88.  Dissimilation. — Nothing  is  more  unstable  than 
protoplasm.  No  sooner  is  new  protoplasm  formed  by 
the  process  of  assimilation  than  it  begins  to  disintegrate, 
forming  various  new  substances,  such  as  cell-walls,  gums, 
resins,  latex,  coloring  matters  (in  flowers  and  other  plant 
parts),  the  perfumes  of  flowers,  the  poisons  of  poisonous 
plants,  and  the  substances  that  give  the  various  flavors 
and  tastes  to  different  kinds  of  plants.  The  process  by 
which  all  such  substances  are  formed  by  protoplasm  is 
called  secretion.  Thus  we  see  that  protoplasm  is  in  a  state 
of  continual  formation  and  disintegration.  The  sum  total 
of  all  these  changes,  both  constructive  and  destructive,  is 
called  metabolism.  It  is  metabolism,  above  everything  else, 
that  distinguishes  living  from  non-living  matter.  We  also 
see  that  death  is  essential  to  life;  unless  protoplasm  per- 
ishes no  new  protoplasm  can  be  formed.  All  life,  it  is 
true,  comes  from  life;  but  only  on  the  condition  that  that 
which  is  already  living  shall  perish. 

89.  Economic  Value  of  Plant  Secretions. — Many  of 
the  substances  secreted  by  protoplasm  are  commercial 
products.  This  is  notably  true  of  wood,  all  of  which  con- 
sists of  lignified  cell-walls;  of  cork,  which  consists  of  sub- 
erized  cell-walls;  of  the  various  gums,  such  as  gum  arabic, 


NUTRITION  91 

gum  mastic,  and  gum  tragacanth;  of  the  resins;  and  of 
turpentine,  rubber,  vegetable  dye-stuffs,  perfumes,  and 
various  other  articles  of  commerce.  Other  plant  secre- 
tions may  play  an  important  role  in  practical  agricul- 
ture, in  connection  with  the  rotation  of  crops,  as 
indicated  in  paragraph  90(6),  below. 

90.  Rotation  of  Crops. — Closely  connected  with  nutri- 
tion and  secretion  is  the  question  of  crop  rotation  in 
agriculture.  Farmers  have  known  for  ages  that  if  one 
kind  of  plant  is  grown  in  the  same  soil  year  after  year  the 
yield  is  greatly  diminished.  Under  such  conditions,  for 
example,  the  yield  of  wheat  will  diminish  from  25  or  30 
bushels  to  12  or  15  bushels  per  acre,  and  also  deteriorate 
in  quality.  Various  hypotheses  and  theories  have  been 
proposed  from  time  to  time  to  account  for  this  fact,  but 
only  three  of  these  theories  are  here  noted,  as  follows: 

(a)  Nutrition  Theory. — We  have  seen  above  that  grow- 
ing plants  withdraw  from  the  soil  various  so-called  mineral 
''nutrients,"  in  solution  in  the  soil-water.  These  are 
essential  to  the  healthy,  vigorous  growth  of  the  plant. 
Different  kinds  of  plants  absorb  these  compounds  in 
different  proportion,  and  one  theory  of  crop  rotation  is 
based  upon  this  fact.  It  is  argued  that,  by  following  one 
kind  of  crop  with  another,  different  demands  are  made  on 
the  soil,  and  the  compound  of  which  the  soil  was  im- 
poverished or  ''exhausted"  by  the  first  crop  is  renewed 
by  capillary  action  from  lower  or  adjacent  regions.  Its 
renewal  is  also  hastened  by  the  application  of  suitable 
fertilizer.  Especially  is  this  true  in  the  case  of  nitrogen, 
which  is  renewed  by  alternating  with  non-leguminous 
plants,  leguminous  crops,  whose  root-tubercle  organisms 
renew  the  nitrates,  as  explained  in  paragraph  82.     This 


g2  THE  VEGETATIVE    FUNCTIONS    OF   PLANTS 

theory  was  advocated  by  the  great  agricultural  chemist, 
Liebig.  Undoubtedly  it  is  correct,  as  far  as  it  goes,  but 
a  more  thorough  consideration  of  the  question  indicates 
that  it  is  not  adequate  as  a  complete  explanation. 

(b)  Theory  of  Toxic  Excreta. — A  second  hypothesis  is 
that  advanced  by  certain  investigators  in  the  Bureau  of 
Soils  of  the  United  States  Department  of  Agriculture.^ 
This  is  based  on  the  fact  that  the  roots  of  plants  are  known 
to  excrete  substances  which  are  poisonous  or  toxic  to  the 
species  producing  them.  These  toxic  substances  accumu- 
late in  the  soil  during  a  succession  of  the  same  kind  of 
crops,  and  thus  gradually  render  it  toxic  to  that  kind  of 
plants.  By  following  with  a  succession  of  different  kinds 
of  crops  the  toxic  excreta  of  the  first  are  either  removed 
by  seepage  of  soil-water,  or  destroyed,  either  by  being 
oxidized,  or  by  the  addition  of  other  substances  which 
render  them  harmless.  By  this  theory  the  function  of 
fertilizers  is  not  so  much  to  renew  exhausted  mineral 
''nutrients,"  as  to  render  harmless  the  accumulated 
excreta  of  the  previous  crop. 

(c)  Sanitary  Theory. — This  theory  has  been  carefully 
worked  out  by  Professor  Bolley,  of  the  North  Dakota 
Agricultural  College.  By  thorough  studies  of  the  wheat 
crop  he  has  been  enabled  to  make  the  following  positive 
statements:^  Constant  or  rather  constant  culture  of 
wheat  on  the  same  lands  brings  about  wheat-sickness,  or 
wheat-sick  soil.  Wheat  does  not  thrive  well  in  the 
presence  of  its  own  dead  bodies,  no  matter  how  fertile 
the  soil.     Constant  wheat  cropping  does  not  especially 

^  A  similar  hypothesis  was  advanced  by  A.  P.  DeCandolle  in  his 
Physiologic  Vegelale,  Paris,  1832. 

2  The  phraseology  of  Professor  Bolley  (North  Dakota  Agric.  Coll. 
Bull.  107),  is  closely  followed. 


NUTRITION  93 

exhaust  or  use  up  the  mineral  "nutrients"  more  rapidly 
than  a  succession  of  the  different  kinds  of  crops,  nor  does 
it  introduce  into  the  soil  any  permanent  excrement  toxic 
to  wheat.  It  does,  however,  tend  to  introduce  with  the 
seed,  stubble,  roots,  et  cetera  a  number  of  kinds  (at  least 
five)  of  parasitic  fungi  that  cause  diseases  of  the  wheat 
plants.  These  fungi  destroy,  blight,  and  rot  off  the  roots 
of  the  plants,  and  live  internally  in  the  straw  and  the 
seeds.  The  accumulation  of  these  fungi  in  fertile  soils 
brings  about  the  condition  of  wheat-sickness,  ''wheat- 
tired"  soil.  The  fungi  attack  the  roots,  leaves,  stems, 
young  developing  grains,  and  seedlings,  and  the  value  of 
crop-rotation  lies  in  growing  a  series  of  different  kinds  of 
crops  that  do  not  transmit  or  bear  each  other's  diseases. 
Crop-rotation  is  not  primarily  to  conserve  the  fertility  of 
the  soil,  but  is  a  sanitary  measure,  tending  to  eradicate 
contagious  disease.  The  reproductive  bodies  {spores)  of 
these  fungi  are  carried  from  field  to  field  and  persist  in  the 
field  for  some  time,  but  lose  their  vitality  during  the  few 
seasons  when  other  crops  are  being  cultivated.  It  is  thus 
seen  that  one  farmer,  by  careless  methods  of  agriculture, 
may  not  only  suffer  a  loss  of  yield  of  his  own  crops,  but 
may  also  infect  his  neighbors'  fields.  In  addition  to  crop- 
rotation,  this  trouble  may  be  reduced  or  avoided  by  care- 
fully sterilizing  all  seed,  before  sowing,  by  soaking  them 
in  a  weak  solution  of  formaldehyde,  and  also  by  sterilizing 
the  soil  in  a  similar  manner.  The  validity  of  this  theory 
is  based  upon  extended  studies  of  wheat,  oats,  barley, 
and  flax:  it  doubtless  holds  true  also  for  other  crops. 

These  three  theories  are  all  based  upon  thorough  experi- 
mental investigation,  and  it  is  probable  that  all  three 
contribute  to  a  rational  basis  for  the  rotation  of  crops. 


CHAPTER   VIII 
FERMENTATION 

91.  Importance  of  Fermentation. — In  Chapter  VII 
(paragraph  78)  reference  was  made  to  enzymes,  which 
have  the  power  of  causing  marked  chemical  changes  in 
other  substances  without  being  thereby  permanently 
transformed  or  used  up  themselves.  The  number  of 
different  kinds  of  enzymes  now  known  is  very  great,  and 
it  is  probable  that  further  investigation  will  reveal  still 
others  not  now  recognized.  One  of  the  most  interesting 
and  illuminating  results  of  their  study  is  the  revelation 
of  the  fact  that  one  or  more  kinds  of  ferments  or  enzymes 
are  produced  by  every  living  cell  (plant  or  animal),  and 
that  life  itself  involves,  and  is  in  large  measure  depend- 
ent  upon,  a  series  of  fermentations.  This  truth,  which  is 
becoming  more  and  more  firmly  established  by  scientific 
research,  was  recognized  as  early  as  1839  by  Schwann, 
one  of  the  founders  of  the  cell-theory.^  His  famous 
work,  '' Microscopical  Researches,"  contains  the  following 
passage: 

"I  have  been  unable  to  avoid  mentioning  fermentation, 
because  it  is  the  most  fully  and  exactly  known  operation 
of  cells,  and  represents,  in  the  simplest  fashion,  the 
process  which  is  repeated  by  every  cell  of  the  living 
body."  In  fact,  a  knowledge  of  enzymes  and  fermentation 
is  necessary  in  order  to  understand  some  of  the  most 

^Cf.  p.  15. 

94 


TERMENTATION 


95 


/f^l 


fundamental  processes  of  plant  physiology.  Fermen- 
tation is  most  commonly  associated  in  our  minds  with 
yeast. 

92.  Yeast. — Practically  everyone  is  acquainted  with 
yeast,  which  was  the  earliest  recognized  agent  of  fermen- 
tation. We  are  now  most  familiar  with  it  in  the  form 
of  small  cakes,  purchased  at 
the  grocer's  for  use  in  making 
bread  and  other  "raised" 
dough.  Our  grandparents 
bought  it  in  liquid  form  from 
the  local  baker;  and  in  brew- 
eries and  large  bakeries  it  is 
used  in  the  liquid  form  in  mak- 
ing beer  and  bread.  If  a  small 
piece  of  a  "compressed  yeast" 
cake,  about  the  size  of  a  pea 
seed,  is  placed  with  a  little 
sugar  and  water  in  a  fermenta-      ^     ,,     ^ 

.  /    .  \  •  ^'^^-  ^^- — Fermentation-tube, 

tion-tube    (Fig.   66),  and    set  in    /,  level  of    fermenting    liqu|d 

a  warm  place  the  mixture  will 

soon  begin  to  "work,"  and  tiny 

bubbles  of  gas  will  be  seen  rising  in  increasing  numbers 

to  the  top  of  the  tube.     The  process  which  gives  rise  to 

these  bubbles  is  alcoholic  fermentation. 

93.  Conditions  Necessary  for  Alcoholic  Fermentation. — 
If  two  other  tubes  are  prepared  precisely  like  the  first 
one,  except  that  ice-water  is  used  in  one  and  boiling  water 
in  the  other,  and  are  set,  the  first  in  a  cold  place  {e.g.,  the 
refrigerator),  and  the  second  in  a  very  warm  place,  fer- 
mentation will  occur  either  very  tardily  or  not  at  all. 
If  a  third  fermentation-tube  is  set  up  with  sugar  but  no 


space  filled  with  gas  (CO2) 
given  off  by  fermentation. 


96       THE  VEGETATIVE  FUNCTIONS  OF  PLANTS 

yeast,  and  a  fourth  tube  with  yeast  but  no  sugar,  no 
fermentation  will  take  place.  In  other  words,  in  order 
to  have  fermentation  three  conditions  must  be  reahzed: 
(i)  a  ferment,  (2)  something  for  the  ferment  to  act  upon, 
(3)  suitable  external  conditions.^ 

94.  Products  of  Alcoholic  Fermentation. — If  we  place 
a  large  quantity  of  the  fermenting  mixture  in  a  deep 
glass  cylinder,  and  cover  over  the  top  so  as  to  hinder  the 
escape  of  the  gas  given  off,  the  gas  will  collect  in  quantity 
in  the  space  above  the  liquid.  If,  after  fermentation  has 
been  allowed  to  proceed  vigorously  for  a  few  hours,  we 
insert  in  the  cylinder  a  lighted  splinter  of  wood  or  a  lighted 
candle,  the  flame  will  at  once  go  out,  showing  that  the 
oxygen  of  the  air,  which  supports  combustion,  has  largely 
disappeared  and  has  been  replaced  by  another  gas. 
The  test  with  the  flame  should,  of  course,  be  made  also  at 
the  very  beginning  of  the  experiment,  to  show  that  the  air 
above  the  Hquid  will  support  combustion  before  fermenta- 
tion has  begun. 

95.  Carbon  Dioxide  Formed. — In  order  to  ascertain 
what  gas  has  taken  the  place  of  oxygen,  we  may  next 
insert  a  fine  wire,  bent  into  a  small  circular  loop  at  the 
end,  and  dipped  in  lime-water.  A  film  of  lime-water 
will  extend  across  the  space  enclosed  by  the  loop.  Lime- 
water  has  the  characteristic  property  of  turning  milky 
in  the  presence  of  carbon  dioxide,  and  in  this  test  the 
film  of  lime-water  will  at  once  turn  white  or  milky,  show- 
ing that  the  gas  given  off  by  fermentation  is  carbon  dioxide. 
It  is  the  formation  of  bubbles  of  this  gas  in  bread  dough 
that  causes  the  dough  to  become  ''light"  and  to  ''rise." 

^  The  student  may  devise  an  experiment  of  his  own  to  ascertain  whether 
or  not  light  is  necessary  to  fermentation. 


FERMENTATION  97 

96.  Alcohol  Formed. — If  a  large  quantity  of  fermenting 
liquid — as  much  as  a  pint  or  a  quart — is  boiled,  and  the 
vapor  that  first  comes  off  is  condensed  to  a  liquid,  this 
liquid  will  have  the  characteristic  odor  of  alcohol,  and 
will  burn  with  a  pale,  almost  colorless  flame.  It  is,  in 
fact,  alcohol,  and  it  is  on  this  account  that  this  kind  of 
fermentation  is  called  alcoholic  fermentation,  to  distinguish 
it  from  other  kinds.  Carbon  dioxide  and  alcohol  are, 
therefore,  two  products  of  the  fermentation  of  sugar  by 
yeast.  If  the  proportions  of  yeast,  sugar,  and  water, 
and  the  temperature  are  properly  adjusted,  and  if  the 
fermentation  is  allowed  to  continue  long  enough,  it  will 
be  found  that  nearly  all  the  sugar  has  disappeared,  having 
been  converted  by  the  yeast  into  carbon  dioxide  and 
alcohol.  Not  all  of  the  sugar  will  be  converted  for,  as 
Pasteur,  the  great  student  of  fermentation,  demonstrated, 
small  amounts  of  other  substances  such  as,  for  example,  suc- 
cinic acid  and  glycerine,  are  formed  by  fermentation,  and 
these  finally  begin  to  check  the  activity  of  the  ferment. 

97.  Heat  Produced. — If  a  delicate  and  accurate  ther- 
mometer is  inserted  into  a  fermenting  yeast-mixture,  and 
the  temperature  recorded  from  time  to  time,  it  will  be 
found  that  the  mixture  grows  gradually  warmer,  indicating 
that  fermentation  produces  heat.  The  experiment  will 
succeed  best  if  the  yeast  mixture  is  placed  in  a  Dewar  flask, 
or,  what  amounts  to  the  same  thing,  in  a  'thermos" 
bottle,  which  is  double-walled,  and  thus  retains  more  of 
the  heat  produced  than  does  an  ordinary,  single-walled 
container.  It  is  important  to  keep  in  mind  the  three 
major  results  of  alcoholic  fermentation:  (i)  the  formation 
of  alcohol;  (2)  the  formation  of  carbon  dioxide;  (3)  the  in- 
crease of  temperature.     We  shall  see,  in  the  next  chapter, 

7 


98  THE   VEGETATIVE    FUNCTIONS    OF    PLANTS 

that  the  last  two  are  also  the  results  of  another  important 
life-process  of  plants. 

98.  What  is  Yeast? — For  centuries  men  employed  yeast 
in  baking  and  brewing  without  having  the  remotest  idea 
as  to  what  it  really  is,  or  of  how  it  causes  fermentation. 
This  was  because  they  did  not  inquire  into  the  matter. 
It  was  not  until  1680  that  Leeuwenhoek,  a  Dutch  natu- 
ralist, discovered  that  liquid  yeast  always  contains  tiny 
floating  globules.  It  was  150  years  after  this  that  a 
French  scientist,  Cagniard  de  la  Tour,  discovered  that 
yeast  is  a  living  organism,  and  soon  thereafter  another 
observer,  Turpin,  demonstrated  that  yeast  is  really  a 
plant,  closely  related  to  the  fungi.  Thanks  to  the 
painstaking  work  of  many  other  students,  and  especially 
of  Pasteur,  we  now  have  a  detailed  knowledge  of  the 
structure  and  activity  of  the  yeast  plant.  It  is  related 
to  the  sac-fungi  (Ascomycetes) . 

99.  Microscopic  Appearance. — If  a  drop  of  yeast  mix- 
ture  that  has  been  fermenting  vigorously  is  examined 

under  the  microscope,  the  indi- 
vidual yeast  plants  may  be 
readily  observed  (Fig.  67). 
They  are  seen  to  be  unicellular 
plants,  globular  or  ellipsoidal  in 
form,  of  various  sizes  according 
to   age,   and  devoid  of  chloro- 

FiG.  67.-Cells'  of  yeast  P^l-  The  nucleus  can  be  seen 
{Saccharoynyces  sp.).   Some  of     only  after  the  cell  is   stained. 

the  cells  are  budding.      The      ^^  c  ,1      a  n        mi  i 

clear  spaces  are  vacuoles.  Many  of  the  larger  cells  Will  be 

seen  to  have  small  outgrowths 

or  buds,  also  of  various  sizes  according  to  age.     It  is  by  the 

formation  of  the  buds,  that  is,  by  budding,  that  the  plant 


FERMENTATION  99 

reproduces  itself.  When  the  buds  reach  a  certain  size, 
they  separate  from  the  parent  plant,  continue  to  take  in 
nourishment  from  the  surrounding  liquid  by  osmosis, 
increase  in  size  to  maturity,  and  then  give  rise  to  other 
plants,  by  repeating  the  process  of  budding.^  Under 
favorable  conditions  new  plants  are  formed  very  rapidly 
by  budding,  so  that  in  the  course  of  a  few  hours  the  total 
number  of  yeast  plants  will  have  enormously  increased, 
notwithstanding  the  fact  that  some  of  them  in  the  mean- 
time may  have  died.  This  increase  in  number  may  be 
noted  with  the  naked  eye,  by  observing  the  increase  of 
turbidity  or  opalescence  of  the  yeast-mixture  after  it  has 
stood  for  an  hour  or  more.  The  yeast  cakes  of  commerce 
consist  largely  of  starch  and  millions  of  tiny  yeast  plants, 
skimmed  from  the  surface  of  a  fermenting  liquid,  and  then 
pressed  together. 

100.  The  Active  Agent  in  Fermentation. — After  it  be- 
came recognized  that  the  presence  of  the  yeast  plant  is 
necessary,  in  order  to  have  alcoholic  fermentation,  it  re- 
quired careful  study  before  it  was  discovered  that  if  the 
yeast  cells,  after  being  disintegrated  by  grinding,  are  all 
filtered  out  of  the  mixture,  the  filtered  liquid  still  re- 
tains the  power  to  cause  fermentation.  From  this  it  was 
learned  that  the  active  agent,  or  immediate  cause  of  the 
process,  is  not  the  yeast  itself,  but  some  substance  or 
substances  produced  by  the  yeast.  These  substances  are 
the  real  ferments  or  enzymes,  and  are  secreted  by  the 
living  protoplasm  of  the  yeast.  At  least  three  different 
enzymes  are  known  to  be  produced  by  yeast.     The  one 

^  Another  process  of  reproduction  of  yeast,  by  the  production  of 
endospores,  need  not  here  be  described. 


IOC  THE  VEGETATIVE   FUNCTIONS    OF   PLANTS 

that  is  active  in  converting  sugar  into  carbon  dioxide  and 
alcohol  is  a  zymase,  called  alcoholase. 

101.  How  Enzymes  Work. — It  has  previously  been 
stated  that  enzymes  have  the  ability  to  cause  changes  in 
other  substances  without  themselves  being  altered  or 
consumed  thereby.  The  mystery  of  this  fact  has  never 
been  fully  explained,  but  the  simile  used  by  Huxley  helps 
us  to  form  a  crude  mental  picture  of  the  process.  ''There 
can  be  no  doubt,"  says  Huxley,  "that  the  constituent 
elements  of  fully  98  per  cent,  of  the  sugar  which  has 
vanished  during  fermentation  have  simply  undergone 
rearrangement;  like  the  soldiers  of  a  brigade,  who  at  the 
word  of  command  divide  themselves  into  the  independent 
regiments  to  which  they  belong.  The  brigade  is  sugar, 
the  regiments  are  carbonic  acid,  succinic  acid,  alcohol, 
and  glycerine."  We  may  add  that  the  commanding 
officer  is  the  enzyme,  secreted  by  the  yeast. 

102.  Many  Kinds  of  Enzymes. — Two  kinds  of  enzymes 
have  just  been  mentioned — that  which  converts  starch 
to  sugar  (diastase),  and  that  which  causes  alcoholic 
fermentation  (alcoholase).  In  our  own  bodies  we  are 
familiar  with  the  ptyalin,  or  "animal  diastase,"  of  the 
saliva,  which  can  also  convert  starch  to  sugar,  the  pepsin 
of  the  gastric  juice,  which  can  change  the  insoluble  pro- 
teins of  meat  into  soluble  proteins,  the  pancreatic  juice, 
and  others.  Among  plants  is  found  cytase,  which  can 
liquefy  the  cellulose  of  cell-walls.  It  is  by  this  means  that 
the  delicate  threads  of  fungi  which  grow  on  trees  can  pene- 
trate the  hard,  solid  wood.  The  enzyme,  secreted  by  the 
fungus,  softens  and  liquefies  the  wood,  and  the  delicate 
fungal  thread  may  then  penetrate  with  ease.  When 
leaves  fall  in  the  autumn,  the  final  stage  in  the  process  is 


FERMENTATION  lOI 

the  solution  of  a  portion  of  the  cell-walls  of  a  layer  of  cells 
(the  ahscission-layer)  at  the  place  where  the  leaf  joins  the 
branch.  This  greatly  weakens  the  attachment,  and  the 
leaf  falls,  often  merely  from  its  own  weight.  The  solu- 
tion of  this  tissue  is  caused  by  an  enzyme  secreted  by 
the  cells  adjacent  to  the  abscission  layer.  Many  of  the 
peculiar  effects  produced  by  molds  and  by  bacteria  are 
caused  by  enzymes  secreted  by  these  organisms.  The 
ripening  of  cheese,  the  formation  of  alcohol  in  beer  and 
wine,  the  changing  of  sweet  to  sour  or  "hard"  cider,  the 
turning  brown  of  cut  fruit,  such  as  apples,  when  exposed 
to  the  air,  and  all  processes  of  decay  are  caused  by  fer- 
mentations produced  by  enzymes  secreted  by  bacteria, 
molds,  yeast,  or  other  living  cells. 

103.  The  Significance  of  Alcoholic  Fermentation. — 
From  what  has  just  preceded,  it  is  seen  that  the  various 
processes  of  fermentation  are  useful  or  otherwise  im- 
portant to  mankind,  but  we  must  seek  for  the  real  sig- 
nificance of  alcoholic  fermentation  in  its  use  to  the 
organism  that  secretes  the  ferment.  In  this  connection 
we  must  recall  the  fact  that  all  the  life-processes  of  plants 
and  animals  require  energy.  The  continued  release  of 
this  energy  within  the  cells  usually  makes  necessary  a 
supply  of  free  oxygen  from  the  air;  but  some  organisms 
can  secure  the  necessary  oxygen  by  decomposing  com- 
pounds that  contain  it — such  as  carbohydrates,  proteins, 
and  fats.  Thus  we  must  regard  alcoholic  fermentation 
(in  part,  at  least)  as  a  process  by  which  yeast  and  other 
organisms  or  cells  secure  the  necessary  energy  for  their 
activities  when  deprived  of  the  free  oxygen  of  the  air. 
Alcohol  and  other  complex  compounds  result  because 
the  processes  of  oxidation  are  not  completed,  owing  to 


I02  THE   VEGETATIVE    FUNCTIONS    OF   PLANTS 

the  restricted  supply  of  oxygen  that  can  be  obtained  in 
the  absence  of  air.  If  the  oxidation  processes  could  he 
completed  the  end  products  would  he  carhon  dioxide  and 
water.  Organisms  that  may  continue  to  live  without  a 
supply  of  free  oxygen  are  called  anaerohes}  Either  the 
entire  organism  may  live  anaerobically,  or  this  condition 
may  be  confined  to  one  or  more  of  its  organs,  or  even  to 
a  single  cell  or  group  of  cells.  It  is  possible  for  yeast  to 
live  in  contact  with  free  oxygen,  but  when  this  is  not 
present  it  can  secure  sufficient  oxygen  for  its  needs  by 
alcoholic  fermentation  alone.- 

104.  Relation  of  Fermentation  to  Our  Daily  Lives. — 
Reference  has  already  been  made  to  the  fact  that  such 
diverse  operations  as  the  manufacture  of  all  alcoholic 
drinks  and  the  making  of  bread  are  dependent  upon 
fermentations,  and  we  have  seen  that  the  process  funda- 
mental to  all  life — the  formation  of  carbohydrates  by 
photosynthesis — probably  involves  the  action  of  at  least 
six  different  ferments.  The  active  principles  of  all  the 
digestive  juices  of  our  own  bodies  are  also  enzymes — 
diastase  in  the  saliva,  pepsin  in  the  stomach,  trypsin  in  the 
intestine,  and  so  on.  We  shall  learn,  in  the  next  chapter, 
that  all  muscular  and  mental  activity,  even  life  itself,  is 
dependent  on  the  fermentative  activity  of  enzymes. 

^  fl  +  CL^^  (air)  +  bios  (life)  =  living  without  air. 

2  From  a  reading  of  Chapters  VII  to  IX  the  student  will  learn  that 
the  transformations  wrought  in  other  substances  by  the  catalytic  agents, 
called  enzymes,  are  not  all  of  the  same  nature;  some  (as  alcoholic  fer- 
mentation, and  digestive  processes)  are  destructive,  others  (as  the  poly- 
merizations and  other  transformations  in  photosynthesis,  mentioned  on 
pages  79  and  8o)  are  constructive.  The  author  believes  that  it  con- 
tributes to  simplicity,  without  sacrificing  clear  and  accurate  thinking,  to 
consider  "enzyme"  and  "ferment"  as  synonymous  terms,  and  to  call  all 
activities  of  enzymes  fermentations,  whether  they  are  analytic  or  synthetic. 


CHAPTER  IX 
RESPIRATION 

106.  Anaerobes  and  Aerobes. — In  the  preceding  chapter 
we  learned  that  all  plants  require  energy  for  their  ac- 
tivities, and  that  this  energy  is  derived  by  the  oxidation 
of  substances  within  the  cell.  In  the  case  of  yeast  and 
other  organisms,  when  living  in  an  atmosphere  devoid  of 
free  oxygen,  the  necessary  oxygen  is  obtained  from  com- 
pounds which  contain  it,  by  the  process  of  fermentation, 
brought  about  by  enzymes.  As  is  well  known,  many  organ- 
isms, such  as  man  and  most  other  animals,  and  most 
plants,  cannot  live  in  the  absence  of  free  oxygen;  such 
organisms,  called  aerobes,'^  continually  take  in  oxygen  from 
the  air.  The  using  up  of  oxygen  by  the  cells  creates  the 
need  for  more,  and  in  the  case  of  man  and  other  mammals, 
the  supply  is  obtained  through  the  lungs  by  breathing. 
Plants,  however,  have  no  lungs,  nor  any  organs  that 
correspond  to  lungs.  ^ 

When  oxygen  is  consumed  by  plant  tissues,  its  pressure 
within  the  plant  becomes  less  than  its  pressure  outside 
the  plant,  and  therefore  more  passes  in  through  the 
stomata  and  epidermis  by  the  simple  physical  process  of 
diffusion. 

^  aero  (air)  -1-  be  {bios,  life)  living  in  air. 

2 Leaves  have  sometimes  been  called  "the  lungs  of  plants."  From  our 
study  of  nutrition,  it  will  be  readily  recognized  that  leaves  may  much  more 
appropriately  be  called  the  stomachs  of  plants. 

103 


I04  THE   VEGETATIVE   FUNCTIONS    OF   PLANTS 

106.  Consumption    of    Oxygen    Demonstrated. — The 

exchange  of  gases  between  the  atmosphere  and  the 
interior  of  the  plant  may  readily  be  demonstrated  by  a 
simple  experiment,  as  follows.  Into  each  of  seven  glass 
cyHnders  (Fig.  68)  fit  a  partition  of  wire  gauze,  so  as  to 
divide  the  space  vertically  into  equal  parts.  Into  the 
first  jar  place  nothing,  and  fill  the  space,  on  one  side  only 
of  the  wire  gauze,  in  each  of  the  other  six  jars  respectively, 
with  germinating  seeds  (pea  or  lupine),  living  herbaceous 
stems,  living  roots  (washed  free  from  soil  but  moist  and 
fresh),  green  leaves,  freshly  opening  flower  buds,  and 
(in  the  last  jar)  fresh  mushrooms  or  other  fleshy  fungi. 
Test  the  air  in  the  empty  half  of  each  jar  with  a  lighted 
taper  to  make  sure  that  it  contains  sufficient  oxygen  to 
support  combustion,  and  then  seal  all  the  jars  air-tight 
with  rubber  stoppers,  and  set  them  side  by  side  in  any 
convenient  place  except  in  direct  sunlight.^  At  the  end 
of  12  to  24  hours  again  test  the  air  in  each  of  the  jars  with 
the  lighted  taper.  The  air  in  the  first  jar,  containing  no 
plant  material,  will  still  support  combustion,  but  the 
taper  will  be  extinguished  at  once  when  lowered  into  each 
of  the  other  six  jars.^  This  shows  that  the  air  in  each  of 
these  jars  has  become  poorer  in  oxygen,  and  that  it  does 
not  now  contain  enough  to  support  combustion.^     This 

1  The  heat  of  direct  sunlight  is  unfavorable  to  the  best  results. 

^  If  the  taper  is  not  extinguished  in  the  jar  containing  the  green  leaves, 
the  student  should  be  able,  from  his  knowledge  of  other  plant  processes, 
to  suggest  a  probable  reason  why,  and  to  devise  suitable  modifications  of 
th€  experiment  so  as  to  demonstrate  the  respiration  of  green  leaves.  In 
this  and  other  tests  of  the  air,  the  cork  stopper  should  not  he  removed  any 
longer  than  is  absolutely  necessary,  and  the  lighted  taper  shoidd  he  lowered 
and  removed  quickly. 

^  Does  the  experiment  show  that  there  is  no  oxygen  in  the  jars  in  which 
the  flame  is  extinguished? 


RESPIRATION 


105 


io6 


THE   VEGETATIVE    FUNCTIONS    OF   PLANTS 


observation    justifies    the    inference    that    living    plants 

take  in  oxygen. 
107.  Carbon  Dioxide  Given  Off. — The  air  in  each  of 

the  seven  cylinders  may  next  be  tested  with  lime-water, 

by  pouring  in  not  more  than 
one  or  two  tablespoonfuls,  and 
mixing  it  well  with  the  air  by 
tipping  the  cylinders,  holding 
the  halves  with  the  plant  ma- 
terial uppermost,  and  allowing 
the  lime-water  to  flow  back 
and  forth  a  few  times  from  one 
end  of  the  jar  to  the  other. 
Care  should  be  taken  not  to 
dirty  the  lime-water  by  allow- 
ing it  to  rinse  the  plant  mate- 
rial. After  this  treatment  the 
lime-water  will  be  found  to 
have  turned  milky  in  all  of  the 
jars  except  the  one  containing 
no  plant  tissue.  From  these 
results  we  may  correctly  infer 
that  carbon  dioxide  has  been 
given  off  by  the  plants. 

108.  Heat  Evolved.— That 
heat  is  evolved  during  the 
gaseous  exchange  above 
demonstrated    may    be   illus- 


FiG.  69. — A  simple  calorim- 
eter for  studying  the  tempera- 
ture transformations  in  respira- 
tion. Respiring  seeds  are  placed 
in  the  Dewar  bulb  (double  walled, 
with  a  vacuum  between  the 
walls).  At  the  bottom  of  the 
bulb  is  a  dish  containing  caustic 
potash;  cotton  wool  is  packed 
between  the  thermometer  and 
the  neck  of  the  bulb.  (After 
Ganong.) 


trated  by  placing  a  delicate 
thermometer  into,  say,  the  germinating  seeds.  The  best 
results  will  be  obtained  by  placing  the  plant  material  into 
a  double- walled  Dewar  flask  (or  a  thermos  bottle),  which 


RESPIRATION  IO7 

will  retain  most  of  any  heat  that  may  be  given  off  (Fig. 
69).  If  the  temperature  is  recorded  at  the  beginning  of 
the  experiment,  and  again  at  the  end,  a  rise  of  temper- 
ature will  be  noted.  In  one  experiment,  set  up  with 
germinating  pea  seeds  (air  dry  weight  80  grams)  in  a 
Dewar  flask  as  above  described,  a  rise  of  19.3°  was  ob- 
served within  96  hours. 

109.  Respiration  Versus  Breathing. — In  the  case  of  man 
and  other  animals,  the  exchange  of  gases  and  evolution 
of  heat,  demonstrated  by  the  experiments  described  above, 
are  an  index  of  respiration.  The  process  of  taking  in 
oxygen  and  giving  off  of  carbon  dioxide  by  animals  is 
called  breathing.  It  is  better  to  restrict  the  term  breath- 
ing to  the  mechanical  exchange  of  gases  between  the  lungs 
of  animals  adn  the  external  air,  and  to  confine  the  term 
respiration  to  the  oxidation  processes  of  the  living  pro- 
toplasm. It  will  thus  be  recognized  that  respiration  is  a 
function  of  every  living  cell,  and  that  the  cells  of  our 
fingers,  for  example,  respire  just  as  truly  as  do  those  of  our 
lungs  and  other  organs.  The  lungs,  by  their  mechanical 
expansion  and  contraction,  merely  serve  to  bring  the 
oxygen  of  the  external  air  into  intimate  contact  with  the 
blood,  which  carries  it  to  all  respiring  tissues  of  the  body. 
There  is  no  process  in  plants  comparable  to  this  breath- 
ing. In  the  case  of  some  animals  without  lungs,  certain 
specialized  organs  (in  fishes,  the  gills)  are  continually 
bathed  with  external  oxygen,  which  passes  into  the 
blood  hy  diffusion.  This  more  closely  resembles  the 
process  by  which  oxygen  from  the  air  passes  into  the 
plant  body.  In  other  animals  {e.g.,  earthworms)  there 
are  no  special  organs  for  breathing,  and  the  oxygen  diffuses 
through  the  moist  body-walls. 


I08  THE  VEGETATIVE   FUNCTIONS    OF   PLANTS 

110.  Stomata  and  Gaseous  Exchange. — In  Chapter  IV 
we  described  the  openings  or  stomata  in  the  epidermis  of 
leaves,  through  which  water-vapor  passed  to  the  out- 
side. We  also  learned  in  Chapter  VII  that  the  inter- 
change of  gases  in  photosynthesis  takes  place  through  the 
stomata.     So,    also,    does    the    exchange    of    gases    that 


Fig.  70. — White  birch  {Betiila  populifolia).     Portion  of  a  branch  showing 
the  prominent  lenticels. 


accompanies  respiration.  As  the  oxygen  within  the 
cells  is  consumed  in  respiration,  more  is  absorbed  from  the 
intercellular  spaces.  Thus  its  pressure  becomes  less 
within  the  leaf  than  without,  and  consequently  oxygen 
enters  by  diffusion  through  the  stomata.  At  the  same 
time  the  air  in  the  intercellular  spaces  becomes  richer 


RESPIRATION  IO9 

in  carbon  dioxide  than  the  air  outside,  and  therefore  this 
gas  passes  out,  also  by  diffusion. 

111.  Lenticels. — Many  living  cells  and  tissues  are  more 
deep  seated  than  the  mesophyll  of  leaves,  and  oxygen 
obtains  access  to  these  cells  by  different  ways  in  different 
plants.  Only  one  of  these  cases  may  be  considered  here. 
If  any  young  woody  twig  is  examined,  small  ''dots"  or 
lines  will  be  discovered  on  the  surface.  On  closer  exami- 
nation these  will  be  found  to  be  small  openings  through 
the  bark  (Fig.  70).  They  are  known  as  lenticels.  The 
outer  portion  of  the  bark,  and  the  older,  inner  layers 
of  wood  are  not  alive,  but  the  cambium  layer,  between 
wood  and  bark  is  alive,  and  therefore  needs  a  continual 
supply  of  fresh  oxygen.  This  is  supplied  through  the 
lenticels  in  a  manner  quite  analogous  to  that  by  which 
the  supply  in  the  leaves  is  maintained. 

112.  Respiration  and  Photosynthesis. — The  two  proc- 
esses of  respiration  and  photosynthesis  are  often  con- 
fused, but  in  reality  thay  have  very  little  in  common, 
except  that  both  result  in  an  exchange  of  gases  with 
the  external  air.  But  it  must  be  kept  clearly  in  mind 
that  the  processes  themselves  are  quite  distinct  from 
the  exchanges  of  gases  that  accompany  them,  or  result 
because  of  them.  Photosynthesis  furnishes  carbon  to  the 
plant  in  a  form  available  for  use;  respiration  is  the 
physiological  process  by  which  the  carbon  is  utilized  to 
supply  the  energy  necessary  for  all  life-processes.  The 
result  of  the  two  processes  is  the  continual  income  and 
outgo  of  carbon.  The  carbon  enters  and  leaves  the 
plant  in  the  same  form,  namely  carbon  dioxide.  The 
disintegration  of  carbohydrates  is  also  accomplished  by 
bacterial  decay  and  other  fermentative  processes.     We 


no 


THE    VEGETATIVE    FUNCTIONS    OF    PLANTS 


thus  see  that  there  is  a  continuous  circulation  of  carbon 
in  nature,  known  as  the  carbon  cycle  (Fig.  71).  A  com- 
parison between  the  two  processes  is  shown  in  Table  II. 


Table  II. — Comparison  of  Respiration  and  Photosynthesis 


t.     Photosynthesis 

Changes  inorganic 
matter  into  plant- 
foods  (carbohy- 
drates), which  are 
Assimilated  and 
used  by  the  plant 


To    supply   energy 
for  work, 
To     repair     waste 
(Nutrition), 
In  the  construction 
of  new  parts 
(Development),  in- 
cluding Reproduc- 
tion. 


Photosynthesis 

I.  Takes  place  only  in  cells  contain- 
ing chlorophyll. 
Requires  light. 
CO2  absorbed,  O  set  free. 
Carbohydrates  formed. 
Plant  gains  in  dry  weight. 
Kinetic   energy   of   sunlight   be- 
comes potential  energy. 


All  of  these  proc- 
esses are  depend- 
ent upon  Oxida- 
tion within  the 
cells. 

This  process  of 
oxidation  is 

called.  .    .    . 


2.    Respiration, 

which    involves 
The  taking  in  of 

oxygen, 
The  oxidizing  of 
oxidizable  mat- 
ter, 

The  release  of 
all  products  of 
these  oxidations. 


Respiration 

1.  Takes  place  in  all  active  cells. 

2.  Can  proceed  in  darkness. 

3.  O  absorbed,  CO2  set  free. 

4.  Carbohydrates  consumed. 

5.  Plant  loses  in  dry  weight. 

6.  Potential  energy  becomes  kinetic 
energy. 


113.  Plant  and  Animal  Respiration. — There  is  probably 
no  erroneous  notion  about  plants  more  tenaciously  held, 
nor  more  widespread,  than  the  belief  that  plant  respira- 
tion is  the  reverse  of  animal  respiration.  This  error  is 
due  entirely  to  a  confusion  of  the  two  processes  of  respira- 
tion and  photosynthesis.  From  what  has  preceded,  how- 
ever, it  should  now  be  clear  that  plants  respire  in  the  same 
way  as  animals,  using  up  oxygen  in  the  processes  of  oxida- 
tion within  their  tissues,  renewing  the  supply  from  the 
surrounding  air  (or,  in  anaerobic  respiration,  from  the 
breaking  down  of  chemical  compounds  rich  in  oxygen),  and 


RESPIRATION 


III 


releasing  the  carbon  dioxide  and  other  waste  products 
resulting  from  the  oxidations.  Heat  is  developed  in  both 
plants  and  animals.  The  condensation  of  water-vapor 
from  the  breath  shows  that  water  is  formed  in  animal 
respiration,  and  careful,  delicate   experiments  have  also 


Carbohydrates 
Plant  proteins 
Pats  . 


Fig.  71. — The  carbon  cycle. 


shown  that  water  is  formed  in  plant  respiration.  In 
both  plants  and  animals  respiration  converts  potential 
energy  (in  the  form  of  complex  chemical  compounds)  into 
kinetic  energy — manifest  in  motion,  locomotion,  and  the 
overcoming  of  resistance  of  various  kinds  (that  is,  work). 
The  two  processes  are  compared  in  the  following  table: 


112  THE   VEGETATIVE    FUNCTIONS    OF    PLANTS 

Table  III. — Comparison  of  Plant  and  Animal  Respiration 

Plant  respiration  Animal  respiration 

1.  Oxidations  occur  within  the  tissue i.  Ditto 

2.  Oxygen  taken  in 2.  Ditto 

3.  Carbon  dioxide  given  off 3.  Ditto 

4.  Heat  evolved 4.  Ditto 

5.  Water-vapor  formed 5.  Ditto 

6.  Dry  weight  decreased 6,  Ditto 

7.  Potential  energy  becomes  kinetic 7.  Ditto 

8.  Occurs  in  every  living  cell 8.  Ditto 

9.  Occurs  without  ceasing,  day  and  night 9.  Ditto 

10.  Accomplished  by  respiratory  enzymes 10.  Ditto 

114.  Respiration  and  Fermentation. — Perhaps  one  of 
the  most  surprising  and  interesting  of  all  the  results  of  the 
study  of  respiration  is  the  revelation  of  the  fact  that 
the  real  process  of  respiration  (the  oxidation  of  living  tis- 
sues, as  distinguished  from  the  exchange  of  gases  by 
breathing,  or  otherwise)  is  accomplished  by  enzymes 
known  as  oxidases,  and  is  therefore,  in  reality,  a  form  of 
jer meditation.  In  fact,  the  more  deeply  we  study  all  the 
fundamental  processes  of  living  things,  the  more  it 
seems  to  become  evident  that  every  chemical  process  in 
organisms,  in  fact,  that  life  itself  is  absolutely  dependent 
upon  fermentations.  We  are  brought  face  to  face  with  the 
almost  startling  fact  that  such  commonplace  phenomena  as 
the  ripening  of  fruit,  the  raising  of  dough,  and  the  decay 
of  plant  tissues,  are  conditioned  by  the  same  class  of  sub- 
stances (enzymes),  and  by  the  same  kind  of  processes  that 
underlie  the  digestion  of  food,  the  respiration  of  tissues, 
and  the  thinking  of  human  minds.  And,  moreover,  we 
seem  to  be  led  to  the  odd  conclusion  that  living  organisms 
do  not  respire  because  they  take  in  oxygen,  hut  that  they 
take  in  oxygen  because  they  have  respired. 


CHAPTER  X 
GROWTH 

115.  Definition. — Growth  is  increase  in  size  of  either  the 
organism  as  a  whole  or  of  any  of  its  parts.  By  growth  the 
individual  protoplast  of  a  cell  may  become  more  bulky, 
the  chloroplasts  or  leucoplasts  may  become  larger,  the 
nucleus  bigger,  the  cell-walls  thicker,  the  cell  as  a  whole 
may  increase  in  any  dimension,  and,  as  a  result  of  this, 
the  tissues  and  organs,  and  finally  the  entire  organism, 
may  become  larger.  Growth  does  not  always  involve  the 
whole  organism.  Cell-walls  often  grow  thicker  while  the 
size  of  the  plant  as  a  whole  does  not  alter.  Growth  often 
involves  a  decrease  in  the  size  of  one  or  more  of  the  parts; 
thus,  when  a  potato  ^'sprouts,"  the  tuber  itself,  giving  its 
substance  to  nourish  the  newly  formed  stems,  becomes 
smaller  and  lighter  in  weight,  while  the  stems  increase  in 
size  and  weight.  As  a  whole,  however,  the  potato  plant 
is  growing  (Fig.  60). 

116.  Osmotic  Pressure  and  Growth. — Growth  does 
not  always  involve  increase  in  weight.  If,  for  example, 
the  osmotic  pressure  increases  within  a  turgid  cell,  and  if 
the  cell- walls  are  elastic,  the  cell  will  grow  bigger  in  one  or 
more  dimensions.  Not  only  may  this  growth  involve  no 
increase  in  weight  (as,  for  example,  when  the  increase  in 
osmotic  pressure  within  a  cell  is  due  merely  to  the  chang- 
ing of  starch  to  sugar),  but  may  even  be  accompanied  by 
loss  of  weight  on  account  of  waste  products  being  given 

8  113 


114  THE   VEGETATIVE    FUNCTIONS    OF   PLANTS 

off  by  the  growing  cell.     The  immediate  cause  of  all  growth 
is  osmotic  pressure. 

The  great  amount  of  force  often  exerted  by  young 
growing  organs,  due  to  the  osmotic  pressure  within  their 
cells,  is  strikingly  illustrated  in  Fig.  72,  showing  the 
rupturing  of  a  concrete  pavement  by  young  fern  leaves. 


Fig.  72. — The  rupturing  of  concrete  by  the  growth  of  young  leaves  of  the 
ostrich  fern  {Onoclea  SlriUhiopteris  Hoflfm.).     (After  Stone.) 

117.  Experiment  in  Growth. — The  relation  between 
osmotic  pressure  and  growth  may  be  demonstrated  by  a 
very  simple  experiment,  illustrated  in  Fig.  73.  P  is  a 
portion  of  the  herbaceous  stem  of  any  convenient  plant, 
fastened  securely  at  one  end  to  an  iron  clamp  (C),  lying 
at  the  bottom  of  a  glass  jar  (J).  The  upper  end  of  the 
stem  is  attached  by  a  small  thread  to  the  short  arm 
of  an  index  (I),  the  opposite  end  of  which  may  move  up 
and  down  over  a  graduated  scale  (S).  If  the  jar  is  filled 
with  a  solution  of  common  salt  water,  water  will  pass  out 
of  the  plant  tissue  by  exosmosis.  This  will  reduce  the 
osmotic  pressure  {turgor)  within,  and  the  stem  will  shorten, 
on  account  of  the  contraction  of  the  elastic  cell- walls, 


GROWTH 


115 


causing  the  long  arm  of  the  index  to  move  up  over  the 
graduated  scale.  If  the  salt-solution  is  now  removed,  and 
the  jar  filled  with  tap  water,  or  better,  distilled  water, 
the  water  will  enter  the  cells  of  the  stem  by  osmosis,  in- 
creasing the  internal  osmotic  pressure  and  turgor  of  each 
cell.  As  a  result  the  stem  as  a  whole  will  elongate  or  grow 
in  length,  thereby  causing  the  index  to  move  down  over 
the  scale. 


Fig.  73.- — Experiment  to  demonstrate  the  relation  between  osmosis 
and  growth  in  length.  /,  jar  containing  water,  and  subsequently  salt- 
solution;  p,  portion  of  leaf-stalk  of  Rhubarb;  /,  index-lever  (portion 
omitted  at  b);  S,  scale.     Explanation  in  text. 

118.  Differential  Growth. — Not  all  the  tissues  of  a 
stem  or  other  part  grow  at  the  same  rate.^  On  this  ac- 
count, and  since  adjacent  tissues  are  closely  united,  those 
which  elongate  or  grow  more  slowly  are  stretched  by  those 
which  grow  more  rapidly.  As  a  result  either  a  state  of 
tension  exists,  or  the  organ  is  distorted,  or  both.  When 
one  epidermis  of  a  leaf  grows  more  rapidly  than  the  other, 
distortion  results,  and  the  leaf  becomes  ''crisped"  or 
crinkled.     This  is  normally  the  case  in  some  plants,  but 

*  The  student  should  endeavor  to  reason  out  an  explanation  for  this. 


ii6 


THE   VEGETATIVE    FUNCTIONS    OF   PLANTS 


in  others  the  crisping  may  denote  a  diseased  condition 
of  the  leaf. 

119.  Tissue-Tension. — If  a  thin  surface  strip  of  tissue 
is  cut  away  for  a  short  distance  from  a  stalk  of   celery, 


^^■mI'  s9H 

VI 

^^^n' -^'  M^l 

^^^^^^B  <"^'  ^^mS^^M 

I'  1 

^^^m  '^-^^iKj^H 

■f  H 

Kl 

H^B 

U 

FiG.^  74. — Longitudinal  tissue-tension  in  leaf-stalk  of  rhubarb.  In  C 
the  strip  of  outer  tissues,  entirely  removed  from  the  main  piece,  is  seen  to 
have  shortened,  showing  that,  before  being  removed,  it  was  in  a  state  of 
longitudinal  tissue-tension,  owing  to  the  fact  that,  in  growth,  the  inner 
tissues  elongated  more  rapidly  than  the  outer,  thus  stretching  the  latter 
lengthwise. 


Fig.  75. — Portion  of  dandelion  scape,  showing  "curls"  resulting  from 
longitudinal  tissue-tension. 


or  the  thick  petiole  of  a  burdock  or  other  leaf,  the  strip 
at  once  curves  outward,  on  account  of  its  longitudinal 


GROWTH 


117 


tissue-tension.  We  are  all  familiar  with  this  phenomenon 
(Figs.  74  and  75).  When  boys  make  whistles  from  young 
willow  twigs  in  spring,  a  cylinder  of  bark  is  removed, 
and  may  be  easily  replaced;  but  if  the  cylinder  of 
bark  becomes  split  lengthwise,  the  edges  cannot  be 
made  to  come  together  around  the  wood  without  consider- 
able stretching.  This  illustrates  transverse  tissue-tension 
(Fig.  76).  If  the  preceding  statements  in  this  chapter 
have  been  understood,  the  student  should  now  be  able 
to  explain  these  phenomena  without  further  assistance. 


Fig.  76. — Portion  of  stem  of  a  willow,  illustrating  transverse  tissue- 
tension.  By  gentle  tapping  and  twisting  the  cambium  layer  has  been 
bruised,  so  that  a  small  cylinder  of  the  bark  was  easily  twisted  oflf. 


120.  Elongation  of  Roots. — In  order  to  ascertain  the 
manner  of  growth  of  roots,  the  root  of  a  young  seedling 
of  lupine,  or  other  plant,  may  be  carefully  marked  with 
dots  or  lines  of  India  ink,  at  intervals  of  i  millimeter, 
beginning  about  i  millimeter  back  from  the  root-tip,  and 
extending  for  a  distance  of  15  to  20  millimeters.  If  the 
root  is  again  observed,  after  having  been  left  to  grow  for 
about  24  hours,  it  will  be  found  that  the  first  six  or  eight 
marks  near  the  tip  have  spread  apart,  those  from  3  to  7 
millimeters  from  the  tip  having  separated  more  than  those 
farther  back  Those  marks  most  remote  from  the  tip 
will  be  found  to  have  separated  very  little  if  at  all  (Fig. 
77).      By    this    simple    experiment  we  learn  that  the 


Ill 


THE  VEGETATIVE  FUNCTIONS  OF  PLANTS 


growth  in  length  of  roots  is  largely  confined  to  a  zone  a 
few  miUimeters  in  length  directly  back  of  the  root-tip. 
It  should  also  be  noted  in  this  connection  that  the  zone 
of  root-hairs  begins  just  back  of  the  zone  of  growth,  no 
root-hairs  being  found  in  the  region  of  elongation. 


Fig. 


77. — Experiment  to  show  the  method  of  growth  in  length  of  a  root. 
A,  24  hours  after  the  condition  shown  in  the  glass. 


121.  Elongation  of  Stems. — The  manner  of  elongation 
of  stems  may  be  ascertained  by  marking  the  internodes^ 
of  a  young  growing  stem  throughout  their  entire  length. 
Two  or  three  adjacent  internodes  near  the  tip  of  the  stem 
should  be  marked.  After  a  period  of  from  24  to  36  hours 
the  marks  will  be  found  to  have  separated,  but,  in  con- 
trast to  the  behavior  of  the  root,  the  marks  will  be  found 
to  have  separated  through  the  entire  extent  of  the  inter- 
node,  and  several  adjacent  internodes  will  be  found  to 
have  elongated  at  the  same  time  (Fig.  78).  This  con- 
tributes to  the  more  rapid  elongation  of  the  stem,  and  is 

^  An  internode  is  the  space  between  two  successive  leaves  on  the  stem. 


GROWTH 


119 


an  advantage  to  the  plant,  since 
the  leaves  are  thereby  more 
rapidly  brought  into  positions 
of  best  exposure  to  air  and  sun- 
light. The  growth  of  several 
internodes  at  the  same  time, 
and  their  elongation  throughout 
their  entire  length,  carries  the 
tip  of  the  stem  forward  with 
much  greater  force  than  if 
growth  were  confined  to  a  short 
zone,  as  in  the  case  of  the  grow- 
ing root.  But  a  more  rapid  and 
forceful  advance  of  the  root-tip 
through  the  soil  might  result  in 
serious  or  fatal  injury,  on  ac- 
count of  the  resistance  and  ob- 
stacles encountered  in  the  soil. 
Thus  the  different  manner  of 
growth  of  stems  and  roots  is  seen 
to  be  of  direct  advantage  to  the 
plant  as  a  whole. 

122.  Growth  of  Leaves. — In 
tropical  climates  leaves  that 
have  once  begun  to  form  con- 
tinue to  grow  until  they  reach 
full  size;  but  in  temperate  cli- 
mates, having  an  alteration  of 
summer  and  winter,  this  is  not 
the  case.  Here  the  leaves  of  any 
given  season  are  all  formed  dur- 
ing the  preceding  growing  sea- 
son,   and    remain    over  winter 


A 

Fig.  78. — Diagram  showing 
mode  of  growth  in  length  of  a 
portion  of  a  stem  of  bindweed 
{Convolvulus) .  A,  stem  with  in- 
ternodes marked  off  into  inter- 
vals of  I  cm.;  B,  the  same  stem 
24  hours  later,  showing  the  rela- 
tive elongation  of  the  various 
internodes.  (Cf.  Fig.  77.)  (Re- 
drawn from  Bonnier  and  Leclerc 
du  Sablon.) 


I20  THE  VEGETATIVE   FUNCTIONS    OF   PLANTS 

in  a  resting  state  in  buds.  The  outer  coverings  or  scales 
of  the  bud  are  modified  leaves  or  parts  of  leaves.  They 
have  almost  or  entirely  lost  their  character  as  foliage 
organs,  and  while  they  are  forming,  their  outer  (dorsal) 
surfaces  elongate  more  rapidly  than  their  upper  (ventral) 
surfaces.     This  causes  them  to  curve  together,  so  as  to 


H^^^^^^^B 

in 

I.  ^fl^^  ^^^^^^BbHIB^II 

i^^^^V^^H 

mm 

SI 

Fig,  79. — Opening    buds    of    horse-chestnut    {AJlsculus  Hippocaslanum). 
(Cf.  Fig.  80.)     (Photo  by  E.  M.  Kittredge.) 

overlap,  and  form  a  protection  to  the  embryonic  stem, 
leaves,  and  other  parts  within  the  bud.  With  the  return 
of  warmth  and  moisture  the  following  spring,  the  bud- 
scales  resume  their  growth,  but  now  their  inner  surfaces 
elongate  more  rapidly  than  their  outer,  reversing  the 
method  of  their  growth  when  forming.  As  a  result  of 
this,  they  begin  to  open  outward.     At  the  same  time  the 


GROWTH 


121 


embryonic  parts  within  the  bud  begin  to  enlarge  and  this 
helps  to  force  the  bud-scales  apart.  The  young  stem-in- 
ternodes  rapidly  elongate,  the  petioles  of  the  leaves  in- 
crease in  length,   and  gradually  the  leaf-blades  expand 


Fig.  8o. — Sapling  of  horse-chestnut  {Msculus  Hippocastanwn),  with  young 
leaves  not  yet  wholly  expanded.     (Cf.  Fig.  79.) 

as   their    cells   become  more  and  more  turgid  (Figs.   79 
and  80). 

123.  Permanent  and  Temporary  Growth. — The  size 
finally  attained  by  stems,  roots,  leaves,  and  other  parts 
is  usually  permanent;  but  some  growth  is  temporary, 
and  certain  tissues  may  manifest  various  alterations  in 


122  THE   VEGETATIVE    FUNCTIONS    OF    PLANTS 


Fig.  8i. — Sensitive  plant  {Mimosa  pudica).     B,  normal  position  of  foliage 
in  light;  A,  position  of  foliage  after  the  plant  was  slightly  shaken. 


Fig.  82. — Oxalis  buplenrifolia.  p,  Petiole;  b,  trifoliolate  blade.  At  the 
left,  leaflets  in  normal  attitude  in  light;  at  the  right,  attitude  of  leaflets  at 
night  or  after  sudden  shock. 


GROWTH 


123 


size  according  to  circumstances.  This  is  illustrated  by  the 
petals  of  flowers  (such  for  example  as  the  tulip),  that  open 
and  close  several  times  before  they  drop  off.  This  motion 
is  caused  by  temporary  fluctuations  of  growth  of  the  upper 
and  lower  surfaces  of  the  petals  In  a  similar  manner  is 
explained  the  change  of  position  of  the  leaflets  of  certain 
plants,  such  as  clover,  oxalis,  bean,  and  others,  at  night 
or  in  cloudy  weather,  and  the  more  rapid  motion  of  the 
leaves  of  the  "sensitive"  and  other  plants  (Figs.  81,  82, 
and  96). 


1 

II 

#€ 

19 

i^s 

•  • 

t 

!?• 

•  « 

1 

Fig.  83. — Structure 'of  seeds.     Bean  '{Phaseoliis),  pea  (Pw^^w),  castor  oil 
{.Ricinus),  lupine  {Lupinus),  Indian  corn  {Zea  Mays). 


124.  Growth  and  Nourishment. — When  a  plant  or 
plant  organ  is  growing,  the  protoplasm  is  constantly 
forming  new  parts,  and  therefore  must  continually  be 
renewed  or  nourished.  The  more  rapidly  new  parts  are 
formed,  the  greater  the  need  for  food.     This  need  is  pro- 


124  THE   VEGETATIVE    FUNCTIONS    OF   PLANTS 

vided  for  in  many  v/ays.  Young  embryo  plants  in  the 
seed  find  a  rich  supply  of  nourishment,  ready  made  by  the 
parent  plant,  stored  within  their  tissues  (as  in  the  bean 
or  pea),  or  around  them  (as  in  the  corn,  or  castor-oil  seed, 
Fig.  83).  They  do  not  have  to  manufacture  their  own 
food  at  first.  Young  and  rapidly  growing  plants  (seed- 
lings and  young  saplings)  have  much  larger  leaves  than 
mature  plants  of  the  same  species  (Fig.  55).  On  this 
account  food  making,  from  the  raw  materials  taken  in 
by  the  plant,  may  proceed  much  more  rapidly.  Plants 
commonly  store  food  in  quantity  where  it  will  be  needed 
in  the  future  by  rapidly  growing  new  parts,  and  such 
storage  organs  usually  become  swollen  by  the  abundance 
of  stored  food.  This  is  illustrated  by  ''potatoes,"  which, 
as  is  well  known,  are  underground  branches  (tubers)  stored 
with  food  for  the  use  of  young  sprouts  when  they  begin 
to  grow  in  the  spring.  All  bulbs  are  to  be  interpreted  in 
the  same  way.  Farmers  recognize  the  need  on  the  part 
of  growing  plants  for  an  abundance  of  food,  when  they 
fertilize  their  fields,  thereby  placing  in  the  soil  a  rich 
supply  of  the  raw  materials  out  of  which  the  growing 
crops  can  manufacture  food  to  meet  their  needs. 


CHAPTER  XI 
ADJUSTMENT  TO  SURROUNDINGS 

126.  Environment. — Not  only  must  plants  be  nour- 
ished, and  respire  in  order  to  live;  they  must  also  be  in 
general  harmony  with  their  surroundings.  The  sum  total 
of  these  surroundings  is  called  the  environment.  Among 
the  factors  of  environment  are  temperature,  water,  light, 
gravity,  air,  electricity,  soil,  animals,  and  other  plants. 
It  will  not  be  possible,  here,  to  study  the  adjustments  of 
the  plant  to  all  of  these  factors,  but  only  to  the  more 
important  ones,  such  as  gravity,  water,  and  light, 

126.  Stimulus  and  Response. — It  is  the  nature  of  the 
various  parts  of  a  plant  to  grow  in  a  certain  definite  rela- 
tion to  their  environment.  Thus,  for  example,  main  stems 
and  roots  normally  grow  parallel  to  the  plumb-line,  while 
their  branches  grow  at  an  angle  to  it ;  foliage-leaves  grow 
naturally  in  the  light,  while  roots  grow  naturally  in  the 
dark.  Any  change  in  the  environment  requires  a  re- 
adjustment on  the  part  of  the  plant,  if  the  latter  is  to 
remain  healthy.  If  the  readjustment  cannot  be  made 
the  given  organ,  or  the  entire  plant,  may  become  un- 
healthy, or  may  die.  The  change  in  the  environment, 
considered  from  the  standpoint  of  its  effect  on  the  plant, 
is  called  a  stimulus;  the  readjustment  or  attempt  at  re- 
adjustment, a  response.  Thus,  if  a  plant  is  growing  at  a 
certain  rate  at  a  certain  temperature,  any  change  in  the 
temperature   becomes   a  stimulus    to   which    the    plant 


126 


THE   VEGETATIVE    FUNCTIONS    OF   PLANTS 


responds  by  growing  either  more  or  less  rapidly,  as  the 

case  may  be. 

127.  Relation  of  Roots  and 
Stems  to  Gravity. — It  is  common 
knowledge  that,  in  general,  roots 
grow  downward,  while  stems 
grow  upward.  If  a  germinating 
seed  of  lupine  is  so  placed  that 
the  axis  of  the  emerging  embryo- 
plant  is  horizontal,  the  position 
of  the  elongating  root  and  shoot 
after  a  given  interval  of  time 
{e.g.  48  hours)  will  be  as  shown 
in  Fig.  84.  The  result  is  the 
same  whether  the  plant  is  placed 
in  diffused  light  or  in  the  dark. 
The  difference  in  the  direction  of 
growth  may  not,  therefore,  be 
attributed  to  the  action  of  light. 
If  the  seedling  is  so  placed  that 
the  only  external  influence  is 
gravity — the  attraction  of  the 
earth — the  result  is  as  shown  in 
the  figure.  We  must,  therefore, 
conclude  that  the  result  is  due 
to  the  earth's  gravitational  at- 


Fig.  84. — Seedling  of  white 
lupine  {Lnpinus  albus),  after  traction, 
having  been  placed  horizon- 
tally in  the  dark  for  48  hours. 
The  hypocotyl  (h)  has  re- 
sponded negatively,  and  the 
root  (r)  positively  to  the  stim- 


The  correctness  of  this  conclu- 
sion may  be  assured  by  subject- 
ing the  seedling  to  another  influ- 
ulus  of  gravity.    The  portion    ence  than  gravity,  but,  like  it, 

(0)    remains   in   the   original  .  ,       ,  .    , 

horizontal  position.  actmg  on  both  root  and  shoot,  at 


ADJUSTMENT  TO   SURROUNDINGS 


127 


right  angles  to  their  long  axis.     Such  an  influence  is  the 
centrifugal  tendency  produced  by  motion  in  a  circle.     If 


-^ 


Fig.  85.— Knight's    experiment,    substituting    centrifugal    "force"    for 
gravity.     (After  Knight.) 

seedlings  are  fastened  to  a  wheel,  with  the  root  and  shoot 
pointing  in  various  directions,  and  the  wheel  rapidly  ro- 
tated, the  growing  root  will  bend  until  it  points  in  the  di- 


128  THE  VEGETATIVE    FUNCTIONS    OF   PLANTS 

rection  of  the  *'puH"  (centrifugal  tendency),  while  the 
shoot  will  curve  in  the  opposite  direction  (Fig.  85). 

The  causal  relationship  between  gravity  and  the  direc- 
tion of  growth  of  roots  and  shoots  was  first  established 
by  the  English  botanist,  Thomas  Andrew  Knight,  who 
devised  the  experiment  illustrated  in  Fig.  78. 

128.  Geotropism. — Careful  thought  about  these  results 
will  make  it  clear  that  the  horizontally  placed  root,  in 
the  first  experiment,  does  not  merely  bend  down  because 
of  its  weight.  If  this  were  so,  we  would  expect  the  shoot 
to  bend  down  also.  The  curving  is  the  response  of  the 
organs  to  the  stimulus  of  the  pull.  The  property  of  an 
organ  by  virtue  of  which  it  may  detect  the  direction  of 
the  pull  of  gravity  is  geotropism.  It  is  thus  seen  that 
geotropism  is  a  particular  kind  of  irritability.  Organs 
which  respond  by  a  curvature  in  the  direction  of  the  pull 
are  positively  geotropic;  those  which  respond  by  a  curvature 
in  the  opposite  direction  are  negatively  geotropic. 

129.  Zone  of  Curvature. — The  following  simple  experi- 
ment shows  that  the  geotropic  curvature  always  takes 
place  in  a  definite  region.  A  germinating  bean  or  other 
seed,  with  the  sprout  Qiypocotyl)  about  15  to  20  milli- 
meters long,  is  pinned  to  a  strip  of  cork,  fastened  to  the 
bottom  of  a  Petri  dish  (Fig.  86).  The  sprout  is  marked 
with  fine  lines  of  India  ink  2  millimeters  apart,  beginning 
2  millimeters  back  from  the  tip,  as  in  the  study  of  growth 
(page  118).  Up  to  this  point  in  the  operations  care  must 
be  exercised  to  keep  the  sprout  as  nearly  parallel  with  the 
plumb-line  as  possible.  By  rotating  the  cork,  or  the 
entire  Petri  dish,  the  sprout  is  now  fixed  at  right  angles 
to  the  plumb-line,  and  the  Petri  dish  covered  and  set  in 


ADJUSTMENT  TO   SURROUNDINGS 


129 


the  dark.^  The  air  in  the  dish  may  be  kept  moist  by  a 
strip  of  damp  filter-paper  placed  around  the  circumference 
of  the  dish,  inside. 


Fig.  86. — Experiment  to  demonstrate  positive  geotropism  in  the  root 
of  a  seedling  of  lupine  {Lupinus  albus).  S,  metal  stand;  P,  Petri  dish, 
with  edges  lined  with  moist  filter-paper.  The  seedling  is  pinned  to  a  strip 
of  sheet  cork.  The  four  views  are  of  the  same  seedling  at  the  successive 
hours  as  indicated.     (Apparatus  after  W.  T.  Bovie.) 


At  the  end  of  12  hours,  more  or  less,  the  sprout,  on  ac- 
count of  its  positive  geotropism,  will  have  responded  to 
the  pull  of  gravity  by  curving  downward,  until  the  por- 


Why  set  in  the  dark? 
0 


I30 


THE   VEGETATIVE    FUNCTIONS    OF    PLANTS 


tion  between  the  curvature  and  the  tip  is  parallel  to  the 
plumb-line.  Examination  of  the  ink-marks  will  show  that 
the  zone  of  curvature  is  the  same  as  the  zone  of  maximum 


Fig.  87. — XcgaLive  geotropism  in 
a  barrigona  palm  {C ol pothrinax 
Wrighlii).  By  some  accident  the 
palm,  when  younger,  had  been  bent 
over. 


Fig.  88. — Mullein  {Vcrhascum 
thapsus)  showing  geotropic  recov- 
ery of  the  terminal  inflorescence, 
after  having  been  bent  over. 


growth  in  length.  The  stimulus  of  gravity  has  modified 
the  distribution  of  the  rate  of  growth  in  such  a  way  that 
the  upper  side  of  the  sprout  has  grown  more  rapidly  than 
the  under  side.     After  the  sprout  has  become  oriented 


ADJUSTMENT   TO   SURROUNDINGS 


131 


in  a  vertical  line,  growth  takes  place  by  equal  amounts, 
in  equal  periods  of  time,  throughout  the  entire  diameter, 
thus  resulting  in  growth  directly  downward.^ 

Geotropic   response  in  nature  is  illustrated  in  Figs. 


Fig. 


-Negative  geotropism  in  a  fleshy  fungus  growing  on  a  treetrunk. 


87-89.     Its  value  is   evident  in  insuring   good  light-ex- 
posure and  the  efficient  distribution  of  seeds  and  spores. 
130.  Problems  to  Solve. — Many  interesting  questions 
now  arise.     How  does  the  root,  for  example,  "  detect '* 

^  Granting,  of  course,  that  all  environmental  conditions  remain  uniform 
on  all  sides. 


132 


THE   VEGETATIVE    FUNCTIONS    OF    PLANTS 


the  fact  that  its  axis  is  not  parallel  to  the  plumb-line? 
Have  the  root  and  the  shoot  a  nervous  system,  or  a  brain? 
How  is  the  bending  accomplished?  These  questions  can- 
not be  discussed  here,  but  they  should  be  given  careful 
thought.  They  lead  us  into  one  of  the  most  fascinating 
realms  of  plant  study. 


Fig.  90. — Phototropic  response  of  a  seedling  of  white  mustard  (Brassica 
alba),  b,  box,  admitting  light  only  through  a  narrow  slit  at  the  right; 
V,  vial;  w,  water-surface;  cl,  cheese-cloth;  sh,  shoot;  r,  root. 


131.  Effect  of  Light  on  Direction  of  Growth.— The 

fact  that  stems  ordinarily  grow  toward,  and  roots  away 
from  the  light,  as  mentioned  above,  is  common  knowl- 
edge.    That  this  is  the  normal  response  of  roots  and  stems 


ADJUSTMENT  TO   SURROUNDINGS 


^33 


to  the  stimulus  of  light  may  be  shown  by  a  very  simple 
experiment.  A  young  seedling  of  mustard,  or  any  other 
convenient  plant,  is  allowed  to  develop  in  complete  dark- 
ness in  ordinary  tap-water,  until  its  root  and  stem  are 


!.;::v. 


f-^      y 


Fig.  91. — Seedling  of  white  mustard  (Brasslca  alba)  showing  the  com- 
bined effect  of  light  and  gravity  on  the  direction  of  growth  of  root  and 
shoot.  The  dotted  Hne  figure  indicates  the  position  of  root  and  shoot  at 
the  beginning  of  the  experiment.  The  change  indicated  was  accomplished 
in  about  48  hours.     (Cf.  Fig.  84.) 


each  about  i  inch  long.  Being  subjected  to  only  the 
influence  of  gravity,  the  root  grows  vertically  downward, 
the  stem  vertically  upward.     If  the  bottle  and  plant  are 


134 


THE   VEGETATIVE    FUNCTIONS    OF   PLANTS 


then  placed  inside  a  covered  box,  to  which  the  Kght  is  ad- 
mitted only  through  a  narrow  sht  in  one  side,  the  stimulus 
of  the  one-sided  illumination  will  be  followed  by  a  curva- 
ture of  the  root  away  from  the  light,  and  of  the  stem 
toward  the  light,  as  shown  in  Fig.  90. 


Fig.  gj.- Mountain  palm  {Gaiissia  sp.),  growing  on  a  steep,  west-facing 
cliff.  The  stems  show  a  phototropic  curvature  toward  the  source  of 
most  abundant  light. 


The  result  of  the  simultaneous  stimulation  of  root  and 
shoot  by  light  and  gravity  is  illustrated  in  Fig.  91. 

132.  Phototropism. — The  property  of  an  organ  by  virtue 
of  which  it  may  detect  the  direction  of  the  source  of  incident 
light  rays  is  phototropism.     Organs  which  respond  by  a 


ADJUSTMENT  TO   SURROUNDINGS  135 

curvature  in  the  direction  of  the  source  of  Hght  are 
positively  phototropic;  those  which  respond  by  a  curva- 
ture in  the  opposite  direction  are  negatively  phototropic. 
Like  geotropism,  phototropism  is  a  special  kind  of  irri- 
tabiHty.  Organs  growing  in  the  light  are,  of  course, 
subject  to  the  influence  of  both  light  and  gravity  at  the 
same  time. 


Fig.  93. — Seedlings  of  t\iQ  whittlM^me.  {Liipinus  alb  us).  At  the  left, 
grown  under  normal  illumination;  at  the  right,  grown  in  darkness.  Both 
cultures  are  of  the  same  age, 


Phototropic  response  on  a  large  scale,  in  nature,  is 
shown  in  Fig.  92. 

133.  Effect  of  Light  on  Rate  of  Growth. — Every  one  is 
famihar  with  the  fact  that  stems  grown  in  darkness,  or 
in   reduced   light,    are   commonly   much   elongated,    and 


136 


THE   VEGETATIVE    FUNCTIONS    OF    PLANTS 


bleached  to  a  pale  yellow  color,  or  even  to  white.  The 
chlorophyll  has  failed  to  develop.  This  condition  is 
called  etiolation.  Early  studies  of  this  phenomenon 
seemed  to  indicate  that  light  retarded  growth  in  length, 
but  more  thorough  and  more  extended  observations 
clearly  showed  that  such  is  not  always  the  case.     The 


I"iG.  94. — Calla  paliistris.  -A ,  Normal  plant,  grown  in  daylight;  B  etiolated 
plant  of  the  same  age,  grown  in  darkness.     (After  D.  T.  MacDougal.) 


stems  of  many  kinds  of  plants  {e.g.,  potato,  pea,  bean) 
undoubtedly  grow  much  longer  in  darkness  than  in  light 
(Fig.  93),  but  in  other  species  the  difference,  if  any,  is 
much  less  (Fig.  94).  A  disturbance  of  the  normal  illumi- 
nation causes  a  general  disturbance  of  the  functions  of  the 
organ  or  of  the  entire  plant,  so  that  not  only  growth,  but 
differentiation  of  tissues  (development),  nutrition,  and 
metabolism  in  general  are  more  or  less  upset,  and  proceed 


ADJUSTMENT  TO   SURROUNDINGS 


137 


138 


THE   VEGETATIVE    FUNCTIONS    OF    PLANTS 


in  an  abnormal  manner.^  Under  conditions  of  normal  illu- 
mination (granted,  of  course,  that  all  other  conditions  are 
normal),  the  physiological  processes  of  the  plant  are  regu- 
lated in  a  normal  manner;  but  when  the  illumination  is 
abnormal,  healthy  regulation  ceases,  and  the  organ  be- 
haves abnormally.  This  abnormal  behavior  includes  the 
failure  of  chlorophyll  to  develop,  and  irregularities  of 
growth.  With  many  plants  the  stems  grow  abnormally 
long  and  slender,  suggesting  that  absence  of  light  favors 
more  rapid  growth  in  length. 


,^Mp^ 


m^'^~ 


^^^km 


M^ji'k^  <:-^m 


Fig.  96. — Potted  plants  of  an  oxalis  showing  the  position  of  the  leaflets 
during  the  day  {A)  and  the  night  {B) — the  so-called  "sleep"  of  plants. 

134.  Relation  of  Leaves  to  Light. — More  than  all  other 
organs  of  the  plant,  the  foliage-leaf  is  developed  and  ad- 
justed with  reference  to  illumination.  Its  form,  dimen- 
sions, and  internal  structure,  and  its  attitude  and  posi- 
tion on  the  stem  are  chiefly  expressions  of  the  surround- 

^  It  is  often  stated  in  "popular"  writings  that  stems  grown  in  darkness 
"reach  for"  or  "seek"  the  hght.  A  careful  consideration  by  the  student 
of  all  that  these  terms  imply,  when  predicated  of  a  plant,  will  lead  at  once 
to  a  recognition  of  their  incorrectness,  and  even  of  their  absurdity. 


ADJUSTMENT  TO   SURROUNDINGS  1 39 

ing  conditions  of  light,  and  clearly  indicate  that  the  chief 


Fig.  97. — House  geranium  (Pelargonium),  showing  back  and  side  views 
of  the  same  plant,  grown  with  the  same  side  always  facing  a  window.  (Cf 
Fig.  98.) 


Fig.  98. — House  geranium  {Pelargonium).  At  the  left,  front  view  of 
the  same  plant  as  shown  in  Fig.  97.  At  the  right,  about  three  days  after 
having  been  reversed,  in  front  of  the  window.  Note  that  only  the  upper, 
younger  leaves  are  properly  adjusted  to  receive  the  light  rays  on  their 
upper  surfaces;  the  lower,  older  leaves  were  not  able  to  change  the  "fixed" 
light  position  they  previously  assumed. 

function  of  leaves  is  to  bring  the  plant  into  suitable  rela- 
tion to  light.     Its  flat  expanded  form  makes  possible  the 


140 


THE   VEGETATIVE    FUNCTIONS    OF    PLANTS 


favorable  exposure  to  light  of  a  large  amount  of  chloro- 
phyll, upon  which  the  plant  is  absolutely  dependent  for 
the  manufacture  of  its  food.  Leaves  that  develop  in 
reduced  light-intensity  (shade),  ordinarily  dispose  of  their 


Fig.  qq. — Geotropic  correlation  among  the  branches  of  a  young  spruce 
tree.  After  the  terminal  bud  of  the  sapling  was  destroyed  one  of  the  lat- 
eral branches  (normally  transversely  geotropic)  became  negatively  geo- 
tropic; ultimately  it  assumed  a  vertical  position  and  became  the  "leader" 
of  the  tree. 

tissue  in  such  a  way  as  to  become  thinner  and  of  larger 
area  than  when  developed  in  more  intense  light.  When 
the  light-intensity  is  increased,  the  paHsade  layer  often 
becomes   double   (Fig.  27).     If  a  leafy  stem  is  bent,  so 


ADJUSTMENT  TO   SURROUNDINGS 


141 


as  to  bring  the  leaf-blade  into  poor  light-exposure,  the 
leaf-stalk,  in  some  plants,  will  bend  and  twist  so  as  to 
restore  the  blade   to  a  suitable  light-relation. 

The  leaves  of  some  plants  (e.g.,  nasturtium)  remain 
adjustable  to  the  direction  of  incident  light  during  their 
active  life  (Fig.  95) ;  the  leaflets  of  legumes  and  some  other 


Fig.  100. — Vertically  growing  branch  of  a  maple,  side  view,  showing 
elongation  of  the  petioles  of  the  lower  leaves,  which  serves  to  prevent  their 
being  shaded  too  much  by  the  leaf-blades  above  them.     (Cf.  Fig.  loi.) 

plants  (Fig.  96)  fold  together  each  night  and  on  cloudy 
days,  thus  manifesting  the  so-called  ''sleep"  of  plants. 
Whether  this  response  is  of  value  to  the  plant  is  not 
entirely  certain.  In  other  plants  {e.g.,  the  house  geranium) 
the  leaves  have  a  fixed  light- position,  and  after  reaching 
maturity  are  not  able  to  readjust  themselves  to  changed 
conditions  of  illumination  (Figs.  97  and  98). 


142 


THE   VEGETATIVE    FUNCTIONS    OF    PLANTS 


135.  Correlation  within  the  Plant. — It  is  of  interest  to 
note  that  the  parts  of  a  plant  are  not  only  sensitive  to  the 
stimulus  of  their  surroundings,  but  are  correlated,  or  ad- 
justed to  each  other.  This  correlation  modifies  their 
reaction  to  external  stimulation.     Thus,  if  the 


''leader," 


Fig.  ioi. — Adjustment  of  leaves,  by  the  lengthening  of  the  petioles,  to 
secure  the  best  illumination,  in  maple.  Top  view  of  same  branch  as  in 
Fig.  I  GO. 

or  main  stem,  of  a  tree  is  destroyed,  one  or  more  of  the 
lateral  branches  will  turn  upward,  in  what  appears  to 
be  an  ''endeavor"  to  perform  the  functions  of  a  leader 
(Fig.  99).  In  other  words,  the  destruction  of  the 
leader  alters  the  mode  of  reaction  of  the  lateral  branches 
to  the  pull  of  gravity. 


ADJUSTMENT  TO  SURROUNDINGS 


143 


136.  Correlation  among  Leaves.— On  a  vertical  and 
equally  illuminated  branch,  as  of  maple,  the  petioles  of 
the  lower  leaves  are  longer  than  those  above  them,  and 
the  stalks  of  opposite  leaves  are  usually  about  equal  in 


the  same  seedlings.  '  """P""^'  ^«-  95,  showing  other  views  of 

length  (Figs.  100  and  loi).  But  on  horizontally  growing 
branches,  the  attitude  of  the  leaves  is  profoundly  altered 
The  length  of  the  petioles,  and  their  attitude  is  such  as 
to  msure  the  placing  of  the  blades  in  positions  for  the 
most  favorable  illumination.  This  often  results  in  a 
marked  twisting  and  bending  of  the  leaf-stalks.    Blades 


144 


THE    VEGETATIVE    FUNCTIONS    OF    PLANTS 


of  opposite  leaves  often  do  not  appear  to  be  opposite  each 
other,  and  are  often  of  very  unhke  size.  Such  an  arrange- 
ment of  the  blades  forms  a  leaf-mosaic  (Figs.  102  and  103). 
If  a  leaf  is  removed  from  a  group,  or  even  if  a  leaflet  is 


t^^-7m''^\^m 


mx^^^ 


^:^i 
v-^^:, 


Fig.  103. — Leaf-mosaic  in  the  Boston  ivy. 

removed  from  the  blade  of  a  compound  leaf,  the  remaining 
leaves  or  leaflets  will  alter  their  positions  with  reference 
to  each  other  so  as  to  occupy  the  space  most  advan- 
tageously and  economically  (Fig.  104). 

137.  Advantages  of  Power  of  Adjustment. — Very  little 
thought  will  enable  one  to  understand  at  once  the  pro- 


ADJUSTMENT   TO   SURROUNDINGS 


145 


Fig.  104. — Effect  of  removal  of  a  leaflet  from  a  palmately  compound 
leaf  {e.g.  Woodbine).  B,  normal  leaf;  C,  after  removal  of  upper  right-hand 
leaflet.  A,  Unbroken  lines  represent  average  normal  position  of  leaflets; 
dotted  lines,  average  position  of  leaflets  after  operation;  barred  line,  posi- 
tion of  leaflet  removed.  (A,  after  Zeleny.) 


Fig.  105. — Motherwort  (Lconurus  Cardiaca),  showing  adjustment  of 
leaves  to  light.  This  plant  grew  at  the  edge  of  shrubbery,  receiving  rela- 
tively little  light  from  behind. 


146  THE  VEGETATIVE   FUNCTIONS    OF   PLANTS 

found  significance  to  the  plant  of  its  ability  so  to  adjust  its 
organs  as  to  bring  them  into  harmony  with  surrounding 
influences.  If  a  stem,  bent  over,  could  not  erect  itself, 
if  leaves  could  not  assume  positions  that  secure  the 
most  favorable  illumination,  if  stems  and  leaves  were  not 
correlated  to  each  other,  most  plants  would  soon  be  out 
of  harmony  with  their  environment  and  would  sicken  and 
die.  Fully  one-half  the  leaves  of  the  motherwort,  illus- 
trated in  Fig.  105,  would  have  been  deprived  of  suitable 
illumination  by  adjacent  plants,  had  they  not  possessed 
this  power  of  adjustment. 

138.  Purpose  of  Part  m. — Chapters  I  to  XI  have  dealt 
with  all  parts  of  the  plant  except  the  flower,  and  all  the 
activities  studied  have  been  primarily  for  the  sake  of  pre- 
serving the  life  of  the  individual  plant.  Flowers  function 
primarily  for  the  race  to  which  the  individual  plant 
belongs;  but  they  can  be  really  understood  only  after  a 
thorough  study  of  the  life  histories  of  some  of  the  lower, 
non-flowering  plants.  The  study  of  life  histories  will  be 
taken  up  in  Part  III. 


PART  HI 

STRUCTURE  AND  LIFE  HISTORIES 


CHAPTER  XII 

LIFE  HISTORY  OF  A  FERN 

139.  Morphology. — In  the  preceding  chapters  we  con- 
sidered various  physiological  processes,  the  primary 
result  of  which  was  to  maintain  the  life  of  the  individual 
plant.  Most  of  those  processes  are  carried  on  by  all 
plants.  Every  one  knows,  however,  that  plants  differ 
widely  from  each  other  in  both  structure  and  habit  of 
life.  In  other  words,  we  recognize  the  fact  of  variation. 
This  means  that  different  plants  solve  the  same  problems 
of  life  in  different  ways.  For  example,  some  plants  expose 
a  large  amount  of  chlorophyll  to  sunlight  by  forming  thin 
leaf-blades  of  relatively  large  area;  while  others,  such  as 
the  cactus,  accomplish  the  same  result  by  developing 
thick,  succulent  green  stems,  and  dispensing  with  leaves 
entirely.  Some  leafy  plants  raise  their  foliage  up  to  the 
light  on  strong  woody  stems,  able  to  stand  alone;  while 
others  secure  this  result  by  climbing  up  on  other  plants. 
In  many  cases  the  organs  of  plants  are  disguised,  appear- 
ing to  be  what,  in  reality,  they  are  not;  stems  may  mas- 
querade as  leaves,  and  leaves  as  stems.  That  phase  of 
botany  which  concerns  itself  with  a  comparative  study  of 
structures,    and   seeks   to   interpret   the   real   structural 

147 


148 


STRUCTURE    AND    LIFE    HISTORIES 


nature  of  an  organ,  no  matter  how  it  may  be  disguised,  is 
termed  the  science  of  form,  or  morphology. 

140.  Life  History. — Every  plant,  in  the  course  of  its 
existence,  passes  through  a  series  of  changes  in  orderly 
sequence.  Like  an  animal,  every  plant  begins  life  as  a 
single  cell,  the  egg,  or  the  equivalent  of  an  egg.  Except 
in  some  of  the  lower  forms,  the   egg   develops  into  an 


Fig.  io6. — A    fei 


(Afiisosorus  hirsukis),  showing    portion  of  the  stem 
above  ground. 


embryo,  and  the  embryo  matures  into  an  adult.  By  a 
series  of  more  or  less  complicated  processes  the  adult 
eventually  gives  rise  to  another  egg,  like  the  one  from 
which  it  came,  thus  completing  one  life-cycle  and  initiat- 
ing another.  These  various  changes  constitute  the  life 
history  of  the  individual.  The  various  stages  of  life 
history  common  to  most  plants  are  nowhere  more  clearly 


LIFE   HISTORY   OF  A  FERN 


149 


Fig.  107. — Portion  of  the  rhizome  of  the  common  brake  {Pteris  aquilina) 
showing  a  cross-section  view  at  the  right.  The  two  dark  areas  are  scler- 
enchyma. 


Fig.   108. — Cross-section  of  the  rhizome  of  the  bracken  fern  {Pteris  aqiii 
Una),  showing  the  tissue  systems.     Greatly  magnified. 


ISO 


STRUCTURE    AND    LIFE   HISTORIES 


illustrated  than  in  the  ferns.     We  shall  therefore  begin 
our  study  of  life  histories  with  that  group  of  plants. 

141.  Description  of  a  Fern  Plant. — The  more  common 
ferns  of  temperate  regions  have  underground  stems  or 
rhizomes  (sometimes  called  root-stocks) ,  so  that  only  the 


Fig.  109. — Tree  ferns  on  the  military  road  between  Cayey  and  Caguas, 
Porto  Rico.     (Photo  by  M.  A.  Howe.) 

leaves  appear  above  ground.-^  The  stem  may  be  branched 
or  unbranched.  When  branched,  the  branches  are  pro- 
duced without  reference  to  the  insertion  of  the  leaves, 
in  contrast  to  the  habit  of  higher  plants  of  forming 
branches  only  in  the  upper  angle  {axil)  between  the  stem 

1  The  leaves  of  ferns  are  often  called  fronds. 


LIFE  HISTORY   OF  A  FERN 


151 


and  leaf-stalk.  There  is  always  a  terminal  bud  at  the 
tip  of  the  fern-stem  (and  of  the  branches  when  any  oc- 
cur) ;  and  the  leaves  are  usually  attached  just  back  of  this 
tip.  The  stems  are  commonly  (though  not  always) 
covered  by  hairs  or  scales  (Fig.  106),  and  on  their  older 
portions,  at  some  distance  back  from  the  tip,  may  be  seen 


C^Mr 


Fig.  1 10. — A,  Upper  epidermis;  B,  lower  epidermis,  of  the  fern,  Drynaria 
meyeniana.     (Camera  lucida  drawing.) 

the  scars,  or  the  ends  of  leaf-stalks,  left  by  old  leaves  that 
have  died  and  fallen  away.  The  rhizome  bears  true  roots 
(Fig.  107),  and  its  tissues  are  differentiated  into  epider- 
mal, fundamental,  mechanical,  and  conducting  systems 
(Fig.  108).  In  tropical  countries  there  are  "tree  ferns," 
with  upright  stems,  and  this  type  of  fern  is  common 
among  the  fossil  plants  of  earlier  geological  ages  (Fig.  109). 


152 


STRUCTURE    AND    LIFE    HISTORIES 


142.  Two  Kinds  of  Fern-leaves. — Careful  examination 
of  the  leaves  of  some  mature  ferns  will  disclose  the  fact 
that  they  are  not  all  alike.  Some  of  them  are  merely 
foliage-leaves,  and  do  not  differ  in  any  essential  point  from 


Fig.   III. — Osmunda  Claytoniana.     Young  sporophylls,  showing  circinate 
vernation.     Note  the  spore-bearing  pinnae. 


the  foliage-leaves  of  higher  plants,  such  as  the  maple  or 
lily;  they  possess  stomata  (Fig.  no),  and  also  resemble 
the  leaves  of  higher  plants  in  their  internal  structure. 
All  fern-leaves,  however,  have  a  very  characteristic  ar- 
rangement in    the   embryonic   or   bud    condition,    being 


LIFE   HISTORY   OF  A  FERN 


153 


coiled  up  from  the  tip.  As  the  leaves  grow  they  unroll, 
and  in  some  ferns,  at  certain  stages,  they  often  closely  re- 
semble the  neck  of  a  violin  (Fig.  in).  The  leaf -blade 
possesses  veins  of  fibro-vascular  bundles  that  pass  down 


Fig.  112. — Portions  of  the  sporophylls  of  two  ferns  to  show  the  sori. 
On  the  left  Poly  podium  piinctaliim  (L,)Sw.;  on  the  right  a  variety  of  Pteris 
longifolia,  with  sporangia  marginal  on  the  pinnules. 


the  leaf-stalk  and  through  the  stem  to  the  roots.  Because 
of  the  possession  of  these  vascular  bundles,  ferns  (and 
all  other  plants  of  which  this  is  true)  are  called  vas- 
cular  plants.     These    leaves    perform    all    the    functions 


154 


STRUCTURE    AND    LIFE    HISTORIES 


performed  by  the  foliage-leaves  of  other  plants,  the  most 
important  of  which  are  photosynthesis  and  transpiration. 
143.  Spore-bearing  Leaves. — The  second  type  of  fern- 
leaf  bears,  on  its  underside,  numerous  "fruit-dots"  or  sori 


Fig.  113. — Sporophylls  of  two  ferns.     At  the  left,  a  species  oi  Poly  podium 
(Phymatodes),  having  no  indusium;  at  the  right,  Diplazium  zelanicum. 


(singular  sorus)  (Figs.  112  and  113).  These  structures 
have  to  do  with  reproduction.  A  single  sorus  of  such  a  fern 
as,  for  example,  Polypodium  irioides,  is  composed  of  a 


LIFE   HISTORY   OF   A   FERN 


155 


cluster  of  tiny  stalked  cases.     The  cases  contain  minute 
•unicellular  reproductive  bodies  called  spores,  and  the  en- 


FiG,  114. — Cross-section  through  the  marginal  sorus  of  a  sporophyll  of 
the  bracken  fern  (Pteris  aquilina).  I,  palisade  layer; /&,  vascular  bundle; 
sp,  sporangium;  in,  indusium.     (Greatly  magnified.) 

tire   structure   is   a  sporangium.     The   place   where   the 
sporangia  are  attached  to  the  leaf  is  the  sporangiophore^ 


Fig.  115. — Cyrtomiiim  falcatum.     Under  (dorsal)  surface  of  a  portion  of 
a  sporophyll,  showing  the  numerous  sori  on  the  pinnae. 

(Fig.  114),  and  over  all  is  often  found  a  thin  membranous 
covering,  the  indusium  (Figs.  114  and  115).  In  some  ferns 
the  indusium  is  lacking,  and  the  sorus  is  naked.     Spore- 

1  Also  called  receptacle. 


156 


STRUCTURE    AND    LIFE    HISTORIES 


bearing  leaves  are  called  sporophylls,  and  plants  that  bear 
sporophylls  are  called  sporophytes. 

144.  Types  of  Foliage-leaf. — In  some  ferns  the  foliage- 
leaf  presents  a  simple,  unbranched  blade,  and  petiole; 
but  in  other  species  the  blade  is  variously  branched.  In 
such  cases  the  larger,  primary  divisions  are  called  pinn(Bj 


Fig.  1 1 6. — Fern  leaves,  showing  various  degrees  of  subdivision  or  branch- 
ing of  the  blade.     A,  Phyllitis;  B,  Polypodium;  C,  Pteris;  D,  Adiantnm. 

and   the  secondary  subdivisions  pinnules.     Illustrations 
of  these  various  types  are  shown  in  Fig.  ii6. 

145.  Sporangia. — As  noted  above  each  sporangium  con- 
sists of  a  spore-case  borne  on  a  stalk  (Fig.  117).  The  struc- 
ture of  the  case  varies  considerably  in  various  groups  of 
ferns,  but  it  usually  possesses  walls  only  one  cell  thick,  with 


LIFE   HISTORY   OF   A   FERN 


157 


a  clearly  differentiated  region,  the  annulus,  composed  of 
cells  whose  radial  and  inner  cell-walls  are  greatly  thick- 
ened.    Various  types   of  spore-cases    are    illustrated    in 


Fig.  117. — Sporangia  of  an  undetermined  species  of  fern;  //,  lip-cells; 
an,  annulus;  st,  stalk;  sp,  mature  spores.  Each  of  the  four  nuclei  in  the 
upper  cells  of  the  stalk  is  in  the  terminal  cell  of  one  of  the  four  rows  of  cells 
that  compose  the  stalk. 

Fig.  118.  Among  the  group  of  ferns  which  are  now  most 
common,  and  to  which  the  bracken  fern  (or  ^' brake"), 
the  maiden-hair  fern,  the  common  polypody,  and  others 


Fig.  118. — Types  of  fern  sporangia.  A,  Loxsoma  Cunninghami;  B, 
Gleichenia  circinata;  C,  Todea  harbara;  D,  Thyrsoptcris  elcgans;  E,  Matonia 
pectinata;  F,  Lygodium  ja ponicum .     (Redrawn  from  various  sources.) 

belong,  the  sporangium  always  originates  from  a  single 
epidermal  cell.  Ferns  whose  sporangia  thus  originate  are 
called  leptosporangiate  ferns.     The  walls  of  their  spore- 


158  STRUCTURE    AND    LIFE    HISTORIES 

cases  are  always  only  one  cell  thick,  and  always  possess 
some  form  of  annulus. 

146.  Spores. — As  the  sporangia  mature  the  spore-case 
itself  becomes  differentiated  into  two  distinct  kinds  of 
tissue,  namely,  sterile  tissue  on  the  outside,  forming  the 
wall,  and  fertile  tissue  within.  In  most  species  of  ferns 
the  fertile  tissue  of  each  sporangium  becomes  organized 
into  16  relatively  large  cells,  rich  in  protoplasm.  As  soon 
as  these  cells  are  mature  they  divide  once,  and  then,  with- 
out resting,  a  second  time,  thus  giving  rise  to  four  cells 
each.  Each  group  of  four  cells  is  called  a  tetrad;  each  cell 
of  a  tetrad  becomes  a  spore.  There  is  always  an  even 
number  of  spores  (usually  64)  formed  in  each  spore-case, 
and  the  cells  from  which  they  are  formed  by  the  two  suc- 
cessive cell-divisions  (tetrad-divisions)  are  spore-mother- 
cells.  The  spores  finally  become  separated  from  each 
other  by  the  dissolving  of  the  middle  layer  (middle  lamella) 
of  the  cell-wall  between  adjacent  spores.  The  solution 
of  this  middle  layer  is  accomplished  by  an  enzyme 
secreted  by  the  cells,  and  which  acts  upon  this  par- 
ticular layer.  The  spores  finally  come  to  lie  dry  and 
perfectly  free  from  each  other  within  the  spore-case. 
Their  purpose  is  to  reproduce  the  plant,  and  especially 
to  multiply  the  number  of  individual  plants. 

147.  Number  of  Spores. — The  number  of  spores  pro- 
duced by  a  vigorous  fern  is  a  great  revelation  to  one  who 
has  never  given  such  matters  careful  thought.  Pro- 
fessor Bower,  of  Glasgow,  has  called  attention  to  this  fact 
in  the  following  words: 

"a  rough  estimate  may  be  made  of  the  numerical  output  of  spores  from 
a  large  plant  of  the  Shield   fern,  as    follows:     In  each  sporangium  48^ 

^  Bower  gives  this  number  as  the  characteristic  output  for  the  species 
Aspidium  Filix-mas.     In  other  species  the  number  may  be  64. 


LIFE   HISTORY   OF  A  FERN 


159 


spores  may  be  formed;  a  sorus  wiU  consist  of  fully  100  sporangia,  usually 
more;  20  is  a  moderate  estimate  of  the  sori  on  an  average  pinna;  there  may 
be  fully  so  fertUe  pinnae  on  one  well-developed  leaf,  and  a  strong  plant 
would  bear  10  fertile  leaves.  48  X  100  X  20  X  50  X  10  =  48,000,000. 
The  output  of  spores  on  a  strong  plant  in  the  single  season  will  thus,  on  a 
moderate  estimate,  approach  the  enormous  number  of  fifty  millions." 

148.  Types  of  Sporophylls.— In  many  ferns  the  leaves 
serve  both  vegetative  and  reproductive  functions  in  about 


Fig.  119— The  cinnamon  fern  (Osmunda  cinnamomea),  showing  foliage 
leaves  and  sporophylls.     (Photo  by  Elsie  M.  Kittredge.) 

equal  degree,  as  in  the  case  of  Polypodium  mentioned 
above.  In  some  species,  however,  there  are  two  kinds  of 
leaves— one  devoted  entirely  to  vegetative  functions,  and 
another  to  the  reproductive,  or  spore-producing  function 
(Fig.  119) ;  between  these  two  extremes  all  grades  of  trans- 
ition are  found  (Fig.  120) .     But  however  widely  the  sporo- 


i6o 


STRUCTURE    AND    LIFE    HISTORIES 


Fig.  120.— Clayton's  fern  {Osmunda  Claytoniana),  showing  sporophylls 
in  the  center,  surrounded  by  foliage  leaves. 

phyll  departs  from  a  foliage-leaf  in  appearance,  it  must, 

nevertheless,  be  regarded  as  morphologically  a  leaf.    As  par- 


LIFE    HISTORY    OF    A   FERN 


i6i 


tial  evidence  of  the  true  foliar  nature  of  sporophylls,  there 
may  be  cited  the  interesting  experiment  of  Atkinson,  who, 
by  removing  the  true  fohage-leaves  just  beginning  to  unfold 


r. 

4 

Fig.  121.-— a  fern  (Tectoria  cicutaria)  that  bears  bulbils  on  both  the 
upper  and  lower  surfaces  of  its  leaves.  Plantlets  develop  from  the  bulbils 
while  they  are  still  attached. 

in  the  spring,  was  able  to  induce  developing  sporophylls 
to  alter  their  character,  and  become  transformed  into 
foliage-leaves.  Similar  results  were  also  obtained  by 
Goebel.     These  experiments  indicate  that  foliage-leaves 


l62 


STRUCTURE    AND    LIFE    HISTORIES 


and  sporophylls  are  very  closely  related  to  each  other, 
and    demonstrate    clearly    that   foUage-leaves    may   be 


Fig.  12  2— Walking  fern  {Camptosorus  rhizophyllus).  The  smaller, 
lower  plant  originated  at  the  tip  of  a  leaf  of  the  larger  plant,  and  one  of  its 
leaves  has,  in  turn,  struck  root. 

produced   by    the   sterilization  of  spore-bearing  leaves. 
The   interesting   question   here    naturally    arises    as   to 


LIFE    HISTORY    OF    A    FERN  1 63 

whether,  in  the  evolutionary  development  of  the  plant 
kingdom,  through  long  geological  ages,  foliage-leaves  have 
in  general  originated  by  the  sterilization  of  spore-bearing 
organs. 

149.  Vegetative  Multiplication. — In  addition  to  re- 
production by  spores,  ferns  may  also  be  propagated  vege- 
tatively  in  at  least  four  ways.  By  one  of  these  methods, 
the  rhizome  is  cut  into  several  pieces,  and  from  every 


Fig.  123. — A  Boston  fern  (Nephrolepis),  reproducing  vegetatively  by 
means  of  runners  or  stolons.  The  parent  plant  is  in  the  round  pot.  (After 
R.  C.  Benedict.) 

piece  that  contains  a  bud  a  new  plant  will  develop.  By 
the  second  method,  the  plant  is  propagated  by  means  of 
bulbils^  which  occur  on  the  foliage-leaves  of  several 
species.  In  the  fern  Tectoria  cicutaria,  bulbils  occur  on 
both  the  upper  and  under  sides  of  the  leaf  (Fig.  121).  These 
bulbils  fall  to  the  ground,  and  under  suitable  conditions  of 
light,  moisture,  and  temperature  give  rise  to  new  fern- 
plants.    One  of  the  ferns  native  to  the  eastern  United  States 


164 


STRUCTURE    AND    LIFE    HISTORIES 


{Cystopleris  bulbijcra)  received  its  specific  name  from  the 
fact  that  it  bears  bulbils.  A  third  method  is  illustrated 
in  the  very  interesting  ''walking  fern"  {Camptosorus 
rhizophyllus) ,  where  the  tips  of  the  long  acuminate  leaves 
rest  upon  the  moist  ground,  take  root,  and  develop  an 
entire  new  plant  at  the  distance  of  the  leaf's  length  from 
the  parent  fern  (Fig.  122).  The  result  of  several  repeti- 
tions of  this  suggested  the  common  name  '^  walking  fern." 
A  fourth  method  is  by  means  of  stolons  or  "runners" 
(Fig.  123). 

150.  Dispersal  of  Spores. — After  the  spores  are  mature 
the  essential  need  is  that  they  become  dispersed,  so  that 


Fig.  124. — Tips  of  two  sporophylls  of  the  fern,  Drynarla  mcyeniana, 
showing  the  large  marginal  sori.  The  black  dots  adjacent  to  the  leaf-tips 
are  spores  projected  onto  white  paper  by  the  snapping  of  the  sporangia. 
The  specimens  were  covered  with  a  bell-jar. 

they  may  find  favorable  conditions  of  moisture,  tem- 
perature, light,  and  soil  for  development;  for,  with  rare 
exceptions,  such  conditions  do  not  obtain  within  the 
spore-case.  Moreover,  if  the  spores  remained  within  the 
sporangia  they  would  be  so  greatly  crowded  that  only  a 


LIFE   HISTORY   OF   A   FERN 


165 


very  small  percentage  of  them  would  be  able  to  develop 
into  new  plants.  When  the  spores  are  ripe,  the  spore-case 
opens,  and  by  various  movements  the  spores  are  expelled. 
That  sporangia  are  able  to  throw  the  spores  to  a 
considerable  distance  may  be  shown  in  a  very  simple 
way  by  placing  a  portion  of  a  sporophyll  with  mature 
sporangia  on  a  sheet  of  white  paper,  with  the  fruit-dots 
uppermost,  and  covering  it  with  a  large  bell- jar.  Within 
a  few  hours  the  scattered  spores  may  be  seen  against 
the  white  background  of  the  paper,  and  the  greatest  dis- 
tance to  which  they  have  been  thrown  may  be  easily 
measured  (Fig.  124). 


Before  germination; 
b,  early  stage,  showing  protonema  (pr.),  and  first  rhizoid  (rh);  c,  d,  e,  f, 
successive  stages  in  the  development  of  the  prothallus. 


151.  Germination  of  Spores. — After  dispersal,  and 
under  favoring  conditions  of  temperature,  moisture 
and  light,  the  spore  begins  to  absorb  water,  and  soon 
commences  to  grow.  As  the  internal  pressure  in- 
creases, the  walls  of  the  spore  are  burst  apart,  and  a  tiny 


l66  STRUCTURE    AND    LIFE    HISTORIES 

tube,  the  germ-tube,  or  protonenia  (first  thread),  begins 
to  develop.  This  process  is  germination.  Shortly,  near 
the  wall  of  the  spore,  a  smaller,  slender  tube  develops  as 
a  branch  of  the  germ-tube  (Fig.  125  ).  This  is  the  first  of 
innumerable  root-like  bodies,  or  rhizoids,  which  will 
help  to  hold  the  new  plant  firmly  to  the  soil,  and  also  serve 
to  take  in  water  and  dissolved  mineral  nutrients. 

152.  The  Prothallus. — Before  the  germ- tube  has  greatly 
enlarged,  it  becomes  divided  into  two  cells,  and  then,  by 


Fig.  126. — Prothallus  of  a  fern.  Archegonia  on  the  (central)  cushion, 
near  the  notch;  antheridia  among  the  rhizoids,  below.  (After  Margaret 
C.  Ferguson.) 

successive  cell-divisions,  into  an  increasing  number. 
Meanwhile  chlorophyll  bodies  begin  to  appear,  but  never 
in  the  rhizoids.  The  final  product  of  these  cell-divisions 
and  growth  is  a  tiny,  flat,  green  body,  often  (but  not 
always)  heart-shaped,  with  a  central  portion,  the  cushion, 
several  cells  thick,  and  a  marginal  part,  the  wings,  of 


LIFE   HISTORY   OF   A   FERN  1 67 

only  one  cell  in  thickness.  Because  of  its  flatness  this 
little  plant  (for  such  it  is)  is  called  a  thallus;  and  because 
it  precedes,  in  the  order  of  reproduction,  the  new  sporo- 
phyte,  it  is  called  the  prothallus  (Fig.  126).  It  is  usually 
possible  to  divide  the  prothallus  into  right  and  left 
halves,  similar  in  shape  and  in  other  characters,  and  hence 
it  is  said  to  possess  bilateral  symmetry. 


CHAPTER  XIII 
LIFE  HISTORY  OF  A  FERN  (Concluded) 

The  prothallus,  as  just  described,  bears  little  resem- 
blance, indeed,  to  the  fern  plant  with  which  we  are  com- 
monly familiar.  In  fact  the  relation  between  the  two  was 
not  understood,  nor  even  suspected,  until  about  1848, 
when  Count  Lesczyc-Suminski,  a  Polish  botanist,  first 
gave  a  connected  description  of  the  life  history  of  the  fern. 
We  shall  now  proceed  to  follow  the  steps  which  lead  from 
the  prothallus  to  the  new  sporophyte. 

153.  Dorso-ventral  Differentiation. — The  appearance 
of  the  first  root-like  body,  or  rhizoid,  was  noted  above. 
As  the  prothallus  develops  the  rhizoids  become  more  and 
more  numerous,  forming  a  mass  of  fine  thread-like  bodies 
on  the  under  side,  opposite  the  notch,  of  the  heart-shaped 
prothallus.  The  presence  of  rhizoids,  and  of  other 
structures  soon  to  be  described,  make  it  easy  to  dis- 
tinguish at  once  the  surface  that  bears  them  from  the 
opposite  surface.  Since  the  surface  bearing  the  rhizoids 
lies  normally  next  to  the  substratum  it  was  called  the 
dentral  surface,  while  the  opposite  surface  was  called 
vorsal.  As  now  used,  the  terms  dorsal  and  ventral  are 
morphological  terms,  and  have  no  reference  to  the  manner 
in  which  the  prothallus  lies.  Normally  the  ventral  surface 
is  the  under  one  and  the  dorsal  surface  the  upper,  but 
the  application  of  the  terms  would  not  be  changed  if 
the    differentiated    prothallus    should    happen,    by    any 

168 


LIFE   HISTORY   OF   A  FERN 


169 


chance,  to  lie  upside  down.  The  dorsal  surface  would 
then  be  the  under  surface,  and  the  ventral  surface  the 
upper  one.  Organisms  or  organs  having  two  such  surfaces 
clearly  distinguishable  are  said  to  have  dorso-ventral 
differentiation.  Among  many  other  structures  thus  dif- 
ferentiated are  foliage-leaves,  sporophylls,  man,  fishes,  and 
other  animals. 

154.  Reproductive  Organs  :  Archegonia.^Examination 
of  the  ventral  surface  of  a  mature  prothallus  with  a  lens 


Fig.  127. — Archegonia  of  a  fern  {Adiantum).  A,  young  archegonium; 
B,  mature;  C,  top  view,  showing  terminal  cells  of  the  four  rows  of  wall 
cells;  V,  wall  of  venter;  e,  egg;  v.cx,  ventral  canal-cell;  n.c,  neck-canal; 
sp,  sperms  entering  the  neck-canal.     A  and  B  in  longitudinal  section. 


will  reveal  near  the  notch  and  on  the  cushion,  several 
tiny  flask-shaped  bodies,  the  archegonia.  Each  arche- 
gonium consists  of  a  wall,  one  cell  thick,  and  contents 
(Fig.  127).  The  neck  projects  away  from  the  surface, 
and  is  usually  slightly  curved,  while  the  remainder,  the 
venter,  is  imbedded  in  the  tissue  of  the  cushion.     As  the 


lyo  STRUCTURE   AND   LIFE   HISTORIES 

archegonium  approaches  maturity  it  is  seen  to  contain 
three  cells;  a  long  neck-canal  cell,  nearly  filling  the  neck, 
an  egg-cell  or  ovum,  filling  the  venter,  and  between  these 
two  a  ventral-canal  cell.  The  egg  is  the  female  reproduc- 
tive cell.  As  it  matures,  the  other  two  cells  become  disin- 
tegrated into  a  mucilaginous  mass  that  fills  the  neck-canal. 
Since  the  archegonia  contain  the  eggs  they  are  the  female 
reproductive  organs. 

155.  Reproductive  Organs :  Antheridia.^ — Search  among 
the    rhizoids    will    reveal    another    class    of    organs,    the 


Fig.  128. — Portion  of  a  cross-section  of  a  prothallus  of  a  fern  (Adian- 
tum),  showing  an  antheridium  (an),  and  sporogenous  cells  within.  (Drawn 
from  preparation  of  E.  W.  Olive.) 

antheridia,  globular  and  also  having  walls  only  one  cell 
thick.  These  are  the  male  reproductive  organs.  At 
maturity  they  contain  a  large  number  of  tiny  motile  cells, 
composed  chiefly  of  a  coiled  nucleus,  and  able  to  swim 
about  in  water  by  the  vigorous  lashing  of  numerous  little 
thread-like  cilia  attached  to  one  end.  These  are  the 
sperms,  or  male  reproductive  cells  (Figs.  128  and  129). 

156.  Fertilization. — Neither  the  eggs  nor  the  sperms  are 
able,  independently,  to  reproduce  their  kind.     In  order 


LIFE  HISTORY  OF  A  FERN 


171 


to  accomplish  this  they  must  unite,  and  the  fusion  of  the 
sperm  and  egg  is  fertilization.  One  of  the  most  significant 
facts  about  fertilization  in  ferns  is  that  free  water  is  re- 
quired, in  order  that  the  sperms  may  reach  the  egg  by  their 
own  locomotion.  When  the  antheridia  and  archegonia 
are  mature,  a  suitable  amount  of  water  (such  as  would 
result  from  a  rain  or  a  copious  dew),  soaking  through  the 


Fig.  129. — Fern  prothallus;  cross-sections  showing  antheridia  (aw), 
sperms  (sp),  and  rhizoids  (rh).  Below  at  the  right  is  a  sperm  (sp)  greatly 
enlarged. 

archegonial  walls,  will  cause  the  mucilaginous  matter  in 
the  neck-canal  to  swell.  This  in  turn  will  rupture  the 
archegonia  at  their  distal  ends,  and  a  portion  of  the  con- 
tents of  the  neck-canal  will  become  extruded,  while  the 
egg  will  remain  in  the  venter.  The  same  conditions  of 
moisture  will  cause  the  rupture  of  the  antheridia,  and  the 
sperms  will  be  set  free  (Fig.  129).    The  mucilaginous  matter 


172 


STRUCTURE    AND    LIFE    HISTORIES 


extruded  from  the  archegonia  contains  a  substance  (malic 
acid,  in  some  ferns)  which  stimulates  the  sperms  to  swim 
toward  it.  This  they  are  enabled  to  do  by  the  free 
external  water.  On  reaching  the  archegonia,  they  enter 
it,  and  swim  down  the  neck-canal  to  the  egg.  The  sperm 
that  first  reaches  the  egg  penetrates  it,  and  passes  through 


Fig.  130. — Fertilization  in  the  fern,  Onoclca.  A,  longitudinal  section 
of  archegonium,  showing  the  egg  in  the  venter,  and  numerous  sperms 
passing  down  the  neck-canal.  B,  an  egg-cell  in  the  venter.  One  sperm 
has  entered  the  nucleus,  three  sperms  have  failed  to  enter  the  egg.  (After 
W.  R.  Shaw.) 

its  cytoplasm  until  it  reaches  the  egg-nucleus,  with  which 
it  fuses,  thus  completing  the  act  oi  fertilization  (Fig.  130). 
As  soon  as  one  sperm  enters  the  egg-cell,  the  latter  at  once 
forms  a.  fertilization-membrane  about  itself,  through  which 
the  remaining  sperms  cannot  enter. 

157.  Nature  of  the  Fertilized  Egg. — It  will  at  once  be 
recognized  that  the  fertiHzed  egg,  resulting  from  a  union 


LIFE  HISTORY   OF  A  FERN 


173 


with  the  sperm,  possesses  a  double  or  diploid  nature.^ 
In  recognition  of  its  dual  nature  it  is  called  the  oosperm 
(egg  and  sperm). ^  The  oosperm,  however,  like  the  un- 
fertilized Qggj  is  still  only  one  cell,  though  its  nucleus  com- 
prises substances  contributed  by  both  egg  and  sperm. 
In  some  cases  the  Qgg  and  sperm  that  unite  in  fertilization 
may  come  from  different  parents;  their  fusion  is  then 
called  cross-fertilization. 


Fig.  131. — Young  embryo  of  a  maidenhair  fern  {Adiantum  concinnum); 
still  surrounded  by  the  archegonium,  which  has  grown  in  size,  L,  leaf, 
S,  stem;  R,  root;  F,  foot.     (After  Atkinson.) 

158.  Development  of  the  Fertilized  Egg. — After  fertili- 
zation the  egg  begins  to  develop,  undergoing  a  series  of 
nuclear   and   cell-divisions,   accompanied   by  increase  in 

^  As  distinguished  from  the  unfertilized  egg,  which  is  of  a  single,  or 
haploid  nature. 

2  The  term  oospore  is  often  used  here,  but  this  term  lacks  the  advan- 
tage of  indicating  the  real  nature  of  the  fertiUzed  egg. 


174  STRUCTURE    AND    LIFE    HISTORIES 

size.  The  cell-wall  of  the  first  division  (in  all  of  the  family 
Polypodiaceae)  is  parallel  to  the  axis  of  the  archegonial 
neck.  The  second  wall,  at  right  angles  to  the  first,  di- 
vides the  oosperm  into  four  cells.  The  beginning  of  these 
divisions  marks  the  beginning  of  the  embryo.  By  further 
cell-divisions  each  of  the  first  four  cells  develops  a  mass  of 
embryonic  tissue.  The  two  cells  on  one  side  of  the  first 
wall  formed  represent,  the  one  the  embryonic  stem,  and 
the  other  the  embryonic  leaf,  or  cotyledon.  One  of  the 
two  cells  on  the  opposite  side  of  the  first  wall,  develops 
into  the  embryonic  root,  while  the  other  develops  into  an 
organ  peculiar  to  the  embryonic  stage,  and  known  as  the 
foot  (Fig.  131).  The  function  of  the  foot  is  to  absorb 
nourishment  for  the  young  embryo  from  the  prothallus, 
by  osmosis.  The  need  of  such  an  organ  becomes  ap- 
parent when  it  is  recalled  that  the  oosperm,  and  conse- 
quently the  embryo,  lie  free  in  the  venter  of  the  arche- 
gonium,  without  any  organic  or  structural  connection 
with  the  prothallus.  This  necessary  connection  is  early 
established  by  the  foot. 

159.  Growth  of  the  Embryo.— As  the  embryo  continues 
to  grow,  the  root  develops  first.  The  advantage  of  this 
will  become  evident  when  we  remember  that  the  primary 
and  most  fundamental  need  of  the  young  plant  is  water, 
which  is  taken  in  by  the  roots.  The  next  most  funda- 
mental need  is  nourishment,  and  as  plant  food  is  manufac- 
tured in  chlorophyll-bearing  organs,  and  usually  in 
leaves,  we  would  expect  the  early  development  of  leaves. 
Such  is  the  case,  the  growth  of  the  first  leaf  being  second- 
ary only  to  that  of  the  root,  and  in  advance  of  the  stem. 
The  development  of  the  stem  follows,  and  finally  spore- 
bearing  leaves   appear    (Fig.    132).     We   then   have   an 


LIFE    HISTORY    OF    A    FERN 


175 


organism  similar  to  that  with  which  we  started — a  full- 
grown  fern-plant,  capable  of  producing  spores,  which  can 
develop  into  prothaUia  again,  with  antheridia  and  arche- 
gonia,  producing  sperms  and  eggs,  and  so  on.  Thus  we 
see  that  the  steps  in  the  life  history  of  a  fern  constitute 
a  life-cycle.  At  whatever  point  or  with  whatever  struc- 
ture we  start,  if  we  follow  the  course  of  development  we 
are  brought  back  again '  to  the  same  point,  or  the  same 
kind  of  structure  with  which  we  began. 


Fig.  132. — ProthaUia  of  a  fern,  i,  Before  the  sporophyte  had  appeared; 
2-5,  with  sporophytes  attached;  I,  cotyledon  or  first  leaf  of  the  sporophyte; 
V,  circinate  vernation  of  a  leaf;  s,  mass  of  soil  adhering  to  the  rhizoids  and 
roots. 


160.  Simpler  Ferns. — In  addition  to  the  leptosporan- 
giate  ferns,  which  have  served  as  a  basis  for  the  general- 
ized description  given  above,  there  is  another  group, 
having  a  more  primitive  type  of  organization.  Repre- 
sentatives of  this  group  include  the  ''moonworts'^  (species 
of  Botrychium,  Fig.  133),  and  the  ''adder's  tongue" 
{Ophioglossum    vulgatunij    Fig.     134).     The    species    of 


176  STRUCTURE    AND    LIFE    HISTORIES 

Botrychiiim  usually  (though  not  invariably)  possess  but 
one  foHage-leaf,   and  a  fertile  spike,  both  of  which  are 


Fig.  133. — Rattlesnake  fern  {Botrychiiim  virginlanmn  {h.)  S\v.), 


more  or  less  branched.     Abnormal  forms    are    not    un- 
common in  which  the  fertile  spike  is  more  or  less  steril- 


LIFE   HISTORY   OF   A   FERN 


177 


Fig.  134.— Adder's  tongue  fern  {Ophioglossum  vulgatum  L.).     R,  ru 


or  stolon. 


1 78  STRUCTURE    AND    LIFE    HISTORIES 

ized,  sometimes  being  entirely  so;  while  in  other  cases 
sporangia  occur  on  the  foliage-leaf.  As  in  the  replace- 
ment of  sporophylls  by  sterile  leaves  in  the  ostrich  fern, 
Onoclea  struthiopteris  (paragraph  148),  these  abnormalities 
indicate  the  close  relationship  between  leaves  and  spore- 
bearing  organs,  and  clearly  show  that  the  latter  may  be 
completely  transformed,  by  sterilization,  into  foliage- 
leaves. 

In  Ophioglossum  the  foliage-leaf  and  spore-bearing  spike 
are  both  unbranched,  the  latter  suggesting  an  adder's 
tongue,  whence  the  name,  Ophioglossum.  In  both  OpJdo- 
glossum  and  Botrychium  the  sporangia  originate  horn  a  group 
of  epidermal  and  sub-epidermal  cells,  and  are  consequently 
imbedded  in  the  surrounding  tissue.  Their  walls  are 
more  than  one  cell  in  thickness,  the  annulus  is  lacking, 
and  they  open  by  a  sHt.  Ferns  of  this  type  are  called 
eusporangiate.  Their  prothahia  are  usually  fleshy  and 
subterranean,  bear  the  antheridia  and  archegonia  on  the 
dorsal  instead  of  on  the  ventral  surface,  and  are  perennial, 
often  living  on  after  the  sporophyte  has  died.  In  general 
the  sporophyte  possesses  less  sterile  tissue  in  proportion 
to  the  fertile  tissue  than  is  the  case  with  the  leptosporan- 
glate  forms.  These  characters  mark  the  group  as  more 
primitive  than  the  leptosporangiate  ferns,  and  they  are 
much  less  numerous,  only  about  100  species  being  known 
from  the  entire  world,  while  of  the  leptosporangiate  ferns 
between  3,000  and  4,000  species  have  been  described. 


CHAPTER  XIV 

FUNDAMENTAL  PRINCIPLES 

161.  Two  Kinds  of  Reproduction. — In  the  two  preced- 
ing chapters  attention  has  been  called  to  three  ways  of  ob- 
taining new  fern-plants,  namely,  by  spores,  by  vegetative 
multiplication,  and  by  fertilized  eggs.  The  first  two 
methods  may  be  grouped  together  as  asexual,  while  the 
second  is  sexual,  as  shown  in  the  following  table. 

Artificial        (slips, 
f  By  the  giving  off  of  i     cuttings,  etc.). 
Asexual,   in-   |     multi-cellular   por-  1  Natural      (tubers, 
volving  cell-   I     tions  or  outgrowths   [   bulbs,  gemmae) . 
divisions         !    of  vegetative  tissue, 
only.  I  By  the  giving  off  of 

special  reproductive 
bodies  of  one  or  few 
cells,  called  spores. 


Reproduction 


Sexual,  in- 
volving cell- 
fusions. 


162.  Vegetative  Multiplication. — Vegetative  multipli- 
cation may  be  accomplished  either  without  or  with  the 
intervention  of  man.  In  the  first  case  the  plant  produces 
special  reproductive  bodies  such  as  tubers,  bulbs,  offsets 
and  stolons,  which  become  separated  from  the  plant  with- 
out assistance,  and  develop  into  new  individuals.  In 
the  second  case  a  similar  result  is  accomplished  through 
the  removal  by  the  gardener  of  portions  of  the  parent 
plant,  such  as  slips,  cuttings,  leaves  {e.g.j  in  the  begonia), 
or  by  bending  branches  over  until  they  touch  the  ground, 
and  there  take  root,  after  which  the  newly  rooted  portion 
may  be  severed  from  the  parent  plant.  This  is  called 
layering.     The  production  of  new  individuals  by  the  arti- 

179 


l8o  STRUCTURE    AND    LIFE    HISTORIES 

ticial  methods  of  the  gardener  is  called  propagation;  but 
between  these  methods  and  the  multiplication  by  special 
bodies,  given  off  spontaneously  by  the  plant,  no  hard  and 
fast  line  can  be  drawn.  Some  plants,  for  example,  be- 
come layered  without  the  gardener's  assistance;  other 
plants  (as  the  willow),  by  self-pruning,  spontaneously 
give  off  branches  from  which  new  plants  may  develop; 
while,  on  the  other  hand,  the  gardener  may  cut  a  tuber, 
such  as  the  "potato"  into  a  number  of  pieces,  from  each  of 
which  a  new  plant  will  develop.  In  this  practice  artificial 
propagation  and  vegetative  multiplication  are  combined. 

163.  Reproduction  by  Spores. — The  essential  fact  about 
a  spore  is  that  it  is  an  individual  cell  or  small  group  of 
cells,  produced  primarily  for  reproductive  purposes, 
given  off  by  the  plant,  and  capable  hy  itself  of  producing 
a  new  individual.  The  essence  of  all  reproduction  is  the 
separation  of  the  reproducing  cell  or  body  from  the  parent 
plant.  If  a  bud  or  a  bulb  remains  attached  to  the  plant 
that  formed  it,  it  produces  only  a  branch  or  other  organ, 
but  not  a  new  individual.  So,  also,  a  spore  must  be  sepa- 
rated from  the  parent  plant  in  order  to  reproduce  the 
latter.  In  many  cases  spores  may  germinate  before  they  are 
set  free,  but  the  separation  must  come  sooner  or  later.  No 
hard  and  fast  line  can  be  drawn  between  spores  and  gemmae. 

164.  Sexual  Reproduction. — In  marked  contrast  to 
reproduction  by  spores,  is  the  reproduction  by  means  of 
sperms  and  eggs,  involving  cell-  and  nuclear-fusions,  known 
as  fertilization.  Eggs  and  sperms  are  called  gametes,'^ 
the  egg  being  the  female  gamete,  the  sperm  the  male 
gamete.  The  diploid  cell,  resulting  from  the  union  of  two 
gametes,  is  called  a  zygote,  and  this  term  is  often  extended 

^  From  the  Greek  word,  7d|Lios  (gamos)j  meaning  marriage. 


FUNDAMENTAL  PRINCIPLES  l8l 

to  apply  to  the  resulting  diploid  organism  through  all 
stages  of  its  development  to  maturity. 

165.  Two  Kinds  of  Generivtions. — A  study  of  the  life 
history  of  the  fern  disclosed  two  distinct  phases  or  genera- 
tions, one  bearing  spores,  and  therefore  called  the  sporo- 
phyte  (spore-bearing  plant) ,  the  other  bearing  gametes  and 
for  that  reason  called  the  gametophyte  (gamete-bearing 
plant).  The  gametophyte  of  the  fern  was  seen  to  be 
entirely  independent  of  the  sporophyte,  capable  of  manu- 
facturing its  own  food  by  means  of  its  own  chlorophyll, 
not  dependent  upon  any  other  plant,  and  in  some  groups 
being  perennial,  living  on  from  year  to  year,  and  giving 
rise  to  sporophytes  that  live  for  only  one  season.  The 
sporophyte,  on  the  other  hand,  is  at  first,  entirely  de- 
pendent upon  the  gametophyte  for  its  nutrition,  living  as 
a  parasite  upon  the  prothallus,  from  which  it  absorbs  its 
nourishment  by  means  of  the  special  organ,  the  foot. 
Gradually,  however,  the  sporophyte  puts  forth  roots, 
capable  of  taking  in  water  and  dissolved  mineral  sub- 
stances from  the  soil,  and  chlorophyll-bearing  organs  (the 
fronds  or  leaves),  capable  of  manufacturing  organic  food. 
As  the  sporophyte  becomes  independent,  the  gameto- 
phyte (with  few  exceptions,  as  noted  above),  perishes. 
A  comparison  of  the  two  generations  shows  that  the 
sporophyte  is  the  much  more  complex  of  the  two,  being 
clearly  differentiated  into  roots,  and  leafy  shoot.  The 
difference  in  the  origin  of  these  two  generations  results  in 
a  very  fundamental  difference  in  the  nature  of  all  the 
cells  in  each.  Since  the  sporophyte  is  derived  from  an 
oosperm  (zygote),  formed  by  the  fusion  of  the  two 
gametes,  all  of  its  cells  are  diploid,  containing  material 
derived  from  both  its  male  and  female  parentage.     The 


l82  STRUCTURE   AND    LIFE   HISTORIES 

gametophyte,  on  the  other  hand,  being  derived  from  a  sin- 
gle reproductive  cell  (the  spore) ,  without  nuclear  or  cell-fu- 
sions, is  composed  of  cells  of  a  single  or  haploid  nature. 

166.  Alternation  of  Generations. — Our  study  of  the 
fern  also  brought  out  another  fact  of  very  fundamental 
importance.  Sporophytes  do  not  produce  sporophytes, 
nor  gametophytes,  gametophytes;  but  there  is  always 
an  alternation  of  generations,  sporophytes  producing 
gametophytes,  and  gametophytes,  sporophytes. 

The  order  of  sequence  in  the  life-cycle  is  as  follows: 

sporophyte — ^•spore — ^gametophyte — ^gametes — >oosperm — >sporophyte. 

The  order  of  structures  and  processes  involved  in  the 
life-cycle  is  as  follows: 

OUTLINE  OF  LIFE  HISTORY  OF  A  FERN 

Gametophyte  (prothallus) 


Antheridium  Archegonium 

i  i 

Sperm  (male  gamete)  Egg  (female  gamete) 

Fertilization 


Oosperm  (zygote) 

4.4. 

Embryo 

ii 

Mature  sporophyte  (mature  zygote) 

4.4. 

Sporophyll 

ii 

Sporangium 

4.4. 

Spore-mother-cell 


Ar  Ar  Ar  Ar 

Spore  Spore  Spore    Spore 

i 

Gametophyte 


Reduction 


FUNDAMENTAL  PRINCIPLES 


183 


The  fact  of  a  cycle  in  the  life  history  is  brought  out 
clearly  in  the  following  diagram : 


Fig.  135. — Diagram  of  life-cycle  of  a  fern. 

167.  Reduction. — Since  the  sporophyte  (descended  from 
the  diploid  oosperm)  has  cells  of  a  double  nature,  resulting 
from  fertilization,  and  since  the  spores  which  give  rise  to 
the  gametophyte  are  of  a  single  (or  haploid)  nature, 
there  must  occur,  at  some  stage  in  the  life  of  the  sporo- 
phyte, a  process  of  reduction,  restoring  the  cells,  made 
diploid  by  fertilization,  to  the  haploid  condition.  Pains- 
taking studies  of  cellular  structure  and  processes  has 
disclosed  the  fact  that  this  reduction  takes  place  during 
the  two  successive  divisions  (tetrad-divisions)  of  the  spore- 
mother-cell,  resulting  in  the  formation  of  four  spores. 
The  diploid  condition  persists  in  all  the  cells  of  the 
sporophyte,  and  through  every  cell-division,  up  to  the  two 
divisions  preceding  spore-formation,  just  as  the  single  or 
haploid  condition  persists  in  all  the  cells  of  the  gameto- 
phyte, up  to  the  very  act  of  fertiHzation. 


i84 


STRUCTURE    AND    LIFE    HISTORIES 


168.  Nature  and  Method  of  Reduction. — In  order 
thoroughly  to  understand  fertihzation  and  reduction  one 
must  have  a  knowledge  of  the  structure  and  behavior  of 
the  nucleus  in  cell-division  and  cell-fusion.     This  subject 


Fig.  136.- — Diagram  illustrating  various  stages  of  indirect  nuclear 
division  (mitosis).  A,  resting  nucleus  of  the  mother-cell;  B,  formation 
of  nuclear  skein  or  spirem;  C,  longitudinal  splitting  of  the  spirem;  D,  the 
chromosomes  (four  in  number)  have  been  formed  by  the  transverse  seg- 
mentation of  the  spirem;  E,  chromosomes  arranged  on  the  equator  of  the 
nuclear  spindle;  F  andC,  early  and  late  anaphase,  the  chromosomes  moving 
to  the  pales  of  the  spindle;  H,  formation  of  daughter  spirems;  /,  resting 
stage  of  the  two  daughter-cells. 


is  too  difficult  and  too  extended  to  be  thoroughly  treated 
in  an  introductory  study,  but  the  sahent  facts  are  as 
follows.  The  nucleus  of  all  cells  comprises  at  least  four 
substances:  nuclear  sap,   a   threadwork   of  linin,   and   a 


FUNDAMENTAL  PRINCIPLES 


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i86 


STRUCTURE    AND    LIFE    HISTORIES 


substance  called  chromatin;'^  all  these  are  enclosed  by  a 
nuclear  membrane.  In  the  non-dividing  nucleus  the 
chromatin  is  distributed  on  the  linin  threads  in  the 
form  of  minute  granules  (Fig.  136).  At  one  of  the  stages 
prehminary  to  nuclear  division  the  linin  network,  with  the 
chromatin,  becomes  transformed  into  a  thickened  skein, 


D \ VI S \0N 


Fig.  138. — Diagram  of  a  cytological  life-cycle,  based  on  a  hypothetical 
fern  with  four  chromosomes  in  the  sporophyte.  The  nuclear  phenomena 
are  based  on  those  of  the  thread- worm  {Ascaris).  Each  chromosome  is 
designated  by  a  characteristic  mark  so  that  it  may  be  traced  throughout 
the  diagram.     (After  R.  F.  Griggs.) 

which  shortly  becomes  split  into  two,  throughout  its  entire 
length.  The  skein  finally  becomes  divided  transversely 
into  a  number  of  double  chromatin  bodies  or  chromosomes. 
The  number  of  these  chromosomes  is  characteristic,  and 
always  the  same  for  each  species  of  plant.     The  nuclear 

^  Because  it  stains  readily  when  treated  with  aniline  dyes. 


J 


FUNDAMENTAL  PRINCIPLES  187 

membrane  then  disappears,  and,  by  a  complicated  mechan- 
ism, not  entirely  understood,  the  two  halves  of  the  chro- 
mosomes are  separated  and  carried  apart  to  opposite  sides 
of  the  cell.  After  this  division  of  the  nucleus,  a  new  cell- 
wall  forms,  dividing  the  entire  cell  into  halves;  new  nuclear 
membranes  develop,  and  the  chromosomes  in  esich  daughter- 
nucleus  become  gradually  retransformed  into  a  resting 
nucleus,  like  the  one  with  which  we  started. 

In  reduction  (Fig.  137)  a  new  resting  nucleus  is  not 
organized  after  the  first  nuclear  division,  but  this  divi- 
sion is  followed  at  once  by  a  second,  or  reducing  division, 
(maiosis)  by  which  the  number  of  chromosomes  in  each 
nucleus  is  reduced  by  one-half.  This  is  the  process  of 
tetrad-division,  by  which  spores  are  formed  from  the 
spore-mother-cells.  The  reduced  number  of  chromosomes 
persists  throughout  the  gametophyte-phase,  including  the 
formation  of  both  egg  and  sperm.  When  the  latter  unite 
the  nucleus  of  the  zygote  will,  of  course,  possess  the  doubled 
number  of  chromosomes,  which  then  persists  throughout 
the  body  of  the  sporophyte  (mature  zygote),  until  the 
stage  of  spore-formation  is  again  reached.  These  facts 
are  shown  diagrammatically  in  Fig.  138. 

169.  Inheritance. — It  is,  of  course,  common  knowledge 
that  men  do  not  gather  grapes  of  thorns,  nor  figs  of 
thistles.  A  given  species  of  fern  always  reproduces  the 
same  species,  and  this  is  true  of  all  plants.  It  requires 
only  a  brief  reflection  to  realize  that  this  must  be  so,  for 
the  beginning  of  every  living  thing  is  always  merely  a 
piece  of  an  antecedent  organism,  the  parent.  The  off- 
spring would,  therefore,  naturally  partake  of  the  nature  of 
its  parent — it  is  a  piece  of  it — was  originally  a  part  of 
it.     Resemblance   between   ancestor   and   descendant  is 


l88  STRUCTURE    AND    LIFE    HISTORIES 

commonly  regarded  as  inheritance,  but  only  a  little 
careful  thinking  will  lead  us  to  see  that,  resemblance 
and  inheritance  are  by  no  means  synonymous.  The  real 
nature  of  inheritance  is  well  illustrated  by  the  inheritance 
of  property  by  a  son  from  his  father.  The  thing  inherited 
is  not  an  external  appearance,  but  a  material  substance 
(land,  buildings,  a  business),  which  is  handed  from  one 
to  another.  So  it  is  in  reproduction.  That  which  one 
generation  of  plants  inherits  from  another  is  the  substance 
of  the  reproductive  cells — the  protoplasm  of  the  spore, 
oosperm,  tuber,  or  bulb — plus  a  certain  characteristic 
organization  of  this  protoplasm,  and  the  effects  of  its  past 
history. 

170.  Inheritance  Versus  Expression.— That  inheritance 
and  expression  are  not  the  same  thing  is  plainly  shown  in 
the  life  history  of  the  fern,  for  the  gametophyte  clearly 
derives  its  living  matter  by  inheritance  from  the  sporo- 
phyte,  and  the  sporophyte,  in  turn,  its  living  matter  from 
the  gametophyte,  and  yet  the  two  generations  look  so 
little  alike  that  men  for  centuries  knew  them  both  with- 
out recognizing  the  fact  that  they  were  merely  two  dif- 
ferent phases  in  the  life  history  of  the  same  species  of 
plant.  So,  often,  among  human  beings,  children  may 
bear  very  little  if  any  resemblance  to  their  parents,  but 
may  closely  resemble  their  grandparents.  Clearly  we 
do  not  inherit  the  color  of  our  eyes  or  hair,  the  shapes  of 
our  noses,  the  peculiarities  of  our  voices,  or  our  mental 
traits  from  our  parents,  nor  even  from  our  more  remote 
ancestors.  What  we  do  inherit  is  a  tiny  particle  of  proto- 
plasm having  a  certain  characteristic  composition,  struc- 
ture, and  past  history.  This  protoplasm  is  capable,  under 
certain    combinations    of    circumstances,    of    developing 


FUNDAMENTAL  PRINCIPLES  1 89 

into  a  mature  organism,  resembling  the  one  from  which 
it  came,  but  under  other  combinations  of  circumstances 
the  external  appearance — the  expression — may  resemble 
that  of  the  parent  only  a  very  little,  or  not  at  all.  In- 
heritance may  therefore  he  defined  as  the  recurrence  in  suc- 
cessive generations,  of  a  similar  cellular  constitution}  Ex- 
pression of  this  cellular  condition  is  greatly  modified  by 
circumstances,  which  are  never  precisely  the  same  for 
any  two  individuals. 

171.  Variation.— The  preceding  sentence  explains,  in 
part,  why  it  is  that  no  two  individuals  are  ever  precisely 
alike — precisely  similar  circumstances  surrounding  de- 
veloping organisms  never  occur  twice ;  that  is,  the  environ- 
ment varies.  Besides  this,  internal  changes  may  take 
place  in  the  reproductive  cells.  For  either  one  or  both  of 
these  reasons,  constant  variation  is  the  rule  for  living 
things.  This  subject  will  be  considered  more  at  length 
in  Chapters  XXXII  and  XXXIII. 

172.  Adjustment  to  Environment. — By  the  term  envir- 
onment is  meant  all  the  circumstances  that  surround  a 
cell,  tissue,  organ,  or  organism  at  any  given  time,  or 
throughout  its  existence.  The  environment  of  tissues 
and  organs  includes  surrounding  tissues  and  organs, 
and  the  environment  of  cells  includes  the  neighboring 
tissues  and  cells.  The  most  essential  thing  in  the 
life  of  every  plant  or  animal  is  to  keep  in  harmony  with 
its  environment.  Every  change  of  environment  neces- 
sitates an  adjustment  on  the  part  of  the  plant  in  order  to 
maintain  this  harmony.  Adjustments  are  most  easily 
made  when  the  plant  is  young  and  plastic,  and  especially 
while  it  is  developing  to  maturity.  If  the  amount  of 
water  in  the  soil  is  diminished  the  young  plant  will  send 

^  Following  Johannsen. 


1 90  STRUCTURE    AND    LIFE   HISTORIES 

its  roots  deeper,  if  light  is  entirely  cut  off  no  chlorophyll 
will  form.  A  leaf,  or  the  prothallus  of  ferns,  is  bilaterally 
symmetrical  because  the  environment  is  uniform  on  all 
sides;  the  same  organs  have  dorso-ventral  differentiation 
largely  because  the  environment  is  unlike  above  and  be- 
low. The  motility  of  sperms  is  an  adjustment  to  water  in 
the  environment.  Thus  variations  in  the  environment 
may  result  in  different  expressions  of  inheritance,  just  as 
variations  in  inheritance  would  be  followed  by  differences 
in  expression,  even  in  an  unchanging  environment.  In 
order  correctly  to  understand  a  plant  nothing  is  more 
necessary  than  to  remember  that  its  characteristics  are 
the  result,  not  of  its  inheritance  alone,  nor  of  its  environ- 
ment only,  hut  of  the  interaction  between  the  two. 

173.  Struggle  for  Existence. — In  Chapter  XII  atten- 
tion was  called  to  the  fact  that  a  moderate-sized  fern  pro- 
duces each  year  about  50,000,000  spores.  If  each  one  of 
these  spores  ultimately  produced  a  mature  fern-plant,  and 
if  we  allowed  only  i  square  foot  of  ''elbow  room"  for  each 
plant,  the  progeny  of  one  parent  only,  in  one  season  would 
require  at  least  50,000,000  square  feet,  or  nearly  1% 
square  miles.  If  each  of  these  plants  in  turn,  produced 
50,000,000  offspring  the  next  season,  the  descendants  of 
only  one  fern  plant  would,  in  only  two  years,  cover  the 
stupendous  area  of  over  83,000,000  square  miles,  or  an 
area  equal  to  that  of  the  North  American  Continent. 
It  has  been  calculated  that  a  single  plant  of  hedge  mustard 
may  produce  as  many  as  730,000  seeds.  If  each  seed 
developed  another  full-grown  plant,  and  if  the  plants  were 
distributed  73  to  each  square  meter,  there  would  be  enough 
mustard  plants  to  cover  an  area  equal  to  2,000  times 
the  dry  surface  of  the  earth.     One  may  easily  imagine 


FUNDAMENTAL  PRINCIPLES  I9I 

the  result  if  all  the  seeds  produced  by  one  of  our  large 
forest  trees  were  able  to  mature.  And  yet  the  total 
number  of  any  given  kind  of  fern,  of  hedge  mustard,  or 
of  forest  tree  does  not  appreciably  change  from  year  to 
year.  The  reason,  of  course,  is  that  not  all  of  the  spores 
and  seeds  produced  are  allowed  to  come  to  maturity. 
The  direct  result  of  the  enormous  number  of  spores  and 
seeds  produced  is  a  struggle  for  existence — for  sufficient  soil, 
water,  light,  and  food  to  insure  a  healthy,  mature  plant. 

174.  Elimination  of  the  Unfit. — As  a  result  of  variation 
certain  individuals  will  succeed  better  than  others  in  the 
struggle  for  existence.  Those  most  poorly  adapted  to 
their  surroundings  will  perish,  and  only  the  more  vigorous 
ones — those  best  adjusted  to  their  surroundings — will 
persist.  The  result  of  this  struggle  for  existence  was 
called  by  Herbert  Spencer  the  ^^  survival  of  the  fittest.'' 
What  really  takes  place  in  nature  is  the  elimination,  by 
death,  of  the  unfit.  Darwin  called  this  natural  selection, 
implying  that  the  result  is  similar  to  that  when  plant 
breeders  select  out  of  a  progeny  the  best  individual  for 
further  breeding.  What  really  takes  place  in  nature, 
however,  is  not  so  much  the  selection  of  the  fittest,  but  a 
rejection  of  the  unfit.  Thus,  among  the  50,000,000 
progeny  of  a  single  fern-plant,  some  are  sure  to  have  a 
weaker  constitution  than  others;  to  develop  a  weaker  root- 
system,  less  chlorophyll  in  their  leaves,  a  less  number 
of  sporophylls  or  spores,  or  to  be  inferior  in  other  ways. 
The  result  will  be  that,  in  the  course  of  only  a  few  years, 
the  descendants  of  the  most  vigorous  or  otherwise  superior 
plants  will  alone  be  left  to  perpetuate  the  race. 

175.  Problems  to  Solve. — In  the  preceding  paragraphs 
we  have  called  attention  to  a  number  of  the  problems 


192  STRUCTURE    AND    LIFE    HISTORIES 

which  arise  from  the  study  of  so  lowly  an  organism  as  a 
fern.  Some  of  these  have  been  partially  solved — prob- 
ably none  of  them  has  been  completely  solved.  In  fact, 
we  may  say  that  our  ignorance  of  life-processes  greatly  ex- 
ceeds our  knowledge.  Very  much  more  remains  to  be 
ascertained  than  has  already  been  found  out;  for  example, 
what  is  protoplasm?  Nobody  really  knows.  We  have 
analyzed  the  substance  chemically,  we  have  carefully 
examined  and  tried  (but  without  complete  success)  to 
describe  its  structure.  We  know  it  is  more  than  merely 
a  chemical  compound.  It  is  a  historical  substance.  A 
watch,  as  such,  is  not.  The  metal  and  parts  of  which  a 
watch  is  made,  have,  it  is  true,  a  past  history;  but  the 
watch  comes  from  the  hands  of  its  maker  de  novo,  without 
any  past  history  as  a  watch.  But  not  so  the  plant  cell. 
It  has  an  ancestry  as  a  cell;  its  protoplasm  has  what  we 
may  call  a  physiological  memory  of  the  past.  It  is  what 
it  is,  not  merely  because  of  its  present  condition,  but 
because  its  ancestral  cells  have  had  certain  experiences. 
We  can  never  understand  a  plant  protoplast  merely  by 
studying  it;  we  must  know  something  of  its  genealogy  and 
its  past  history. 

What  is  the  origin  of  the  sporophyte,  and  how  did 
there  come  to  be  two  alternating  generations?  What  is 
the  meaning  of  fertilization;  what  the  mechanism  and 
laws  of  inheritance?  How  did  there  come  to  be  on  the 
earth  such  plants  as  ferns?  What  was  the  origin  of  Hfe? 
What  is  Hfe?  No  one  can  give  complete  answers  to  these 
questions;  but  the  purpose  of  the  study  of  botany  is  to 
help  fit  us  to  seek  the  answers  intelHgently.  To  those 
who  are  interested  in  problems  of  this  sort,  nothing  can 
be  more  fascinating,  nor  more  profitable. 


CHAPTER  XV 
LIFE  HISTORY  OF  A  MOSS 

176.  Variety  of  Mosses. — There  have  been  described  and 
named  over  12,000  different  species  of  Musci,  or  mosses. 
Obviously,  in  an  introductory  study,  we  can  only  get  a 
glimpse  of  so  large  a  group.  A  comparative  study  of  the 
species  has  led  to  the  recognition  of  three  distinct  orders 
as  follows: 

[  I.  Sphagnales  (the  peat-mosses) 
Musci     2.  Andreasales  (the  black  mosses) 
1  3.  Bryales  (the  true  mosses) 

Of  these  the  Sphagnales  are  considered  the  most  primitive, 
and  the  Bryales  most  highly  developed.  The  Sphagnales 
will  be  considered  first. 

177.  Habitat  of  Sphagnum. — Peat-mosses,  as  the  name 
implies,  grow  in  swamps  and  lake  margins,'  usually  in 
dense  clumps  or  thick  mats,  in  places  forming  the  familiar 
peat-bogs  of  northern  regions.  They  are  usually  of  a  very 
pale  green  color,  often  almost  white,  especially  just 
below  the  top,  and  frequently  with  a  tinge  of  red  or 
yellow. 

178.  Description  of  Sphagnum. — The  plant  consists  of 
an  upright  central  axis  or  stem,  with  a  central,  pith-like 
portion  of  thin-walled  parenchyma  (Fig.  139.)  The  cell- 
walls  of  the  outer  portion,  or  cortex,  are  thicker  and  often 
tinted  with  a  reddish  pigment.     The  cortex  varies  in  thick- 

?  193 


194 


STRUCTURE    AND    LIFE    HISTORIES 


ness  from  four  cells  in  the  main  stem,  to  one  or  two  cells 
in  the  smaller  branches.  The  leaves  are  only  one  cell  thick, 
and  are  densely  crowded  on  the  stem,  having,  at  maturity, 
what  is  known  as  the  two-fifths  arrangement;  that  is, 
if  one  starts  with  a  given  leaf  and  follows  upward  in  a 
spiral  around  the  stem,  he  will  pass  five  leaves  before  he 
comes  to  one  vertically  above  that  with  which  he  started. 


Fig.  139. — Sphagnum  sp.     Upper  portions  of  leafy  plants,  showing 
sporogonia. 


and  in  doing  this  he  will  have  passed  twice  around  the 
stem.  The  leaves  of  sphagnum  never  have  a  mid-rib 
or  other  veins,  and  correlated  with  this  is  the  entire 
absence  of  any  fibro-vascular  bundles  in  the  stem.  This 
is  one  of  the  features  that  marks  the  plant  as  of  lower 
organization  than  the  fern.  The  stem  forms  numerous 
branches,  usually  one  for  every  fourth  leaf,  and  glandular 
hairs  are  usually  met  with  at  the  bases  of  very  young 
leaves  (Figs.  140  and  141). 


LIFE  HISTORY  OF  A  MOSS 
C  G 


195 


Fig.  140. — Sphagnum  cymbifolium,  Ehrb.  A,  B,  C,  cells  from  a 
young  leaf,  X  about  300;  D,  cells  from  a  mature  leaf;  E,  section  of  a 
similar  leaf;  F,  cross-section  of  an  old  stem,  showing  the  thick,  large  celled 
cortex,  X  about  25;  G,  Sclerenchyma  cells  from  the  central  portion  of  the 
stem,  X  about  300.     (After  Campbell.) 


Fig.  141. — Sphagnum  cymbifolium,  Ehrb.  A,  median  longitudinal 
section  of  a  slender  branch;  x,  the  apical  cell;  B,  part  of  a  section  of  the 
same  farther  down,  showing  the  enlarged  cells  at  the  bases  of  the  leaves, 
and  the  double  cortex  (cor);  C,  cross-section  near  the  apex  of  a  slender 
branch;  D,  glandular  hair  at  the  base  of  a  young  leaf;  all  X  525.  (After 
Campbell.) 


196  STRUCTURE    AND    LIFE    HISTORIES 

As  the  leaves  mature,  part  of  their  cells  increase  greatly 
in  size,  and  the  protoplasm  becomes  entirely  transformed 
into  thickenings  of  the  cell-walls,  leaving  the  cells  quite 
empty  of  everything  but  air  and  water.  These  large, 
empty  cells  greatly  mask  the  smaller  ones  containing 
chlorophyll,  and  this  accounts  for  the  pale  color  of  the 
plants.  The  walls  of  the  empty  cells  are  commonly  per- 
forated with  several  pores.  A  similar  type  of  cell  is  also 
developed  in  the  outer  layers  of  the  stem  (Fig.  141). 
These  cells  are  extremely  hygroscopic,  and  absorb  water 
rapidly  and  in  large  quantities,  so  that  the  entire  living 
plant  is  usually  thoroughly  saturated  with  water.  On 
account  of  its  sponge-like  nature,  sphagnum  is  much  used 
by  florists  in  packing  plants  for  shipment,  and  in  other 
ways. 

179.  Sterile  and  Fertile  Branches. — Two  kinds  of 
branches  occur:  sterile  and  fertile.  The  organs  of  repro- 
duction (antheridia  and  archegonia)  occur  only  on  the 
fertile  branches,  but  antheridia  and  archegonia  never  on 
the  same  branch.  In  some  species  they  occur  on  separate 
plants  {dioecious — two  households) ;  in  other  species  on  the 
same  plant  {monoecious — one  household). 

180.  Antheridial  Branches. — The  male  branches,  or 
antheridiophores,  bear  leaves  that  vary  in  color  from 
green  to  yellow  and  red,  and  the  antheridia  occur  in  the 
axils  of  these  leaves  (Fig.  142).  They  consist  of  a  rela- 
tively long  stalk,  composed  of  four  rows  of  cells,  or  less, 
and  bearing,  at  maturity,  the  globular  capsule  containing 
the  sperms  (Fig.  143).  The  sperms  are  coiled,  with  about 
two  complete  turns,  and  bear  two  long  thread-like  cilia  at 
their  anterior  end.  In  locomotion  the  end  bearing  the  cilia 
precedes.     At  the  opposite  or  posterior  end  occurs  a  small 


LIFE  HISTORY  OF  A  MOSS 


197 


appendage  composed  of  starch,  which  ultimately  drops 
off.  It  will  be  seen  at  once  that  the  sperms  of  sphagnum 
differ  from  those  of  ferns  in  having  only  two  cilia  instead 
of  many.  When  the  antheridium  is  ripe  the  cells  swell, 
and  the  capsule  is  thus  forced  open. 


Fig.  142. — Sphagnum.     Photomicrograph  of  a  longitudinal  section  of  an 
antheridial  branch,  showing  five  antheridia.     (Cf.  Fig.  143.) 

181.  Archegonial  Branches. — The  female  branches,  or 
archegoniophoreSy  usually  occur  near  the  upper  end  of 
the  plant,  and  bear  the  archegonia  at  their  tips.  As  in 
the  case  of  the  fern,  each  archegonium  consists  of  a 
neck  (slightly  twisted  in  Sphagnum),  with  neck-canal,  a 
venter,  containing  the  egg,  and  a  basal  stalk,  or  pedicle, 
not  found   in  the  ferns.     Several  archegonia  commonly 


iqS 


STRUCTURE    AND    LIFE   HISTORIES 


Fig.  143. — Sphagnum  acutifoliutn,  Ehrb.  A,  prothallus  (pr),  with  a 
young  leafy  branch  just  developing  from  it;  B,  portion  of  a  leafy  plant;  a, 
male  cones;  ch,  female  branches;  C,  male  branch  or  cone,  enlarged  with 
a  portion  of  the  vegetative  branch  adhering  to  its  base;  D,  the  same,  with 
a  portion  of  the  leaves  removed  so  as  to  disclose  the  antheridia;  E,  an- 
theridium  discharging  spores;  F,  a  single  sperm;  G,  longitudinal  section 
of  a  female  branch,  showing  the  archegonia  (ar);  H,  longitudinal  section 
through  a  sporogonium;  sg^,  the  foot;  ps,  pseudopodium;  c,  calyptra;  sg, 
sporogonium,  with  dome  of  sporogenous  tissue;  ar,  old  neck  of  the  arche- 
gonium;  /.  Sphagnum  squarrosum  Pers.;  d,  operculum;  c,  remains  of  calyp- 
tra; qs,  mature  pseudopodium;  cA,  perichaetium.  (Cf.  Figs.  139  and  142.) 
(From  Schimper.) 


LIFE  HISTORY  OF  A  MOSS 


199 


occurs  together  in  a  group  on  a  single  female  branch.  A 
number  of  enlarged  leaves  surrounding  the  archegonia  con- 
stitute a  perichcBtium.  The  antheridial  and  archegonial 
branches  at  first  occur  close  together  near  the  summit  of 
the  branch,  but  the  branch  often  elon- 
gates in  the  region  between  the  two, 
thus  separating  them. 

182.  Asexual  Multiplication. — One 
of  the  sterile  branches,  near  the  apex 
of  the  plant,  usually  develops  more 
strongly  than  the  others,  and  each  year 
the  old  stem  below  dies  off,  and  the 
young  branch  becomes  established  as  a 
new  plant.  Under  favorable  condi- 
tions young  plantlets,  called  innovation 
branches,  may  develop  on  the  sterile 
branches,  at  the  tip  and  back  from  the 
tip,  strike  root,  and  become  established 
as  independent  plants  (Fig.  144). 

183.  Sexual  Reproduction. — Fertili 
zation  is  accomplished  in  a  manner 
similar  to  that  in  the  fern,  a  film  of 
water  being  required  in  order  that  the 
motile  sperm  may  swim  to  the  neck-canal,  down  which  it 
passes,  to  the  venter  and  into  the  egg,  where  the  two  nuclei 
unite.  Fertilization  probably  occurs,  as  a  rule,  in  winter, 
for  young  embryos  are  usually  found  in  very  early  spring. 
The  first  division-wall  of  the  oosperm  is  horizontal,  or 
nearly  at  right  angles  to  the  axis  of  the  neck,  and  thus 
at  right  angles  to  the  position  of  the  wall  in  the  first  divi- 
sion of  the  leptosporangiate  ferns.  As  the  cell-divisions 
follow  each   other  in  rapid  succession,  the  upper  cells 


Fig.  144. — Sphag- 
num cuspidatum, 
showing  innovation, 
or  short,  branches. 
(After  Schimper.) 


200 


STRUCTURE   AND   LIFE   HISTORIES 


of  the  developing  embryo  become  organized  into  a  large 
globular  spore-case,  containing  a  thick  central  column 
(columella),  surrounded  by  a  dome  of  spores,  and,  out- 
side of  all,  the  wall  of  the  sporangium.     As  the  spore-case 


Fig.  145. — Sphagnum  sp.  Photomicrograph  of  a  longitudinal  section 
through  a  sporogonium  and  portion  of  the  pseudopodium.  op^  operculum; 
r,  annulus;  ca,  wall  of  capsule;  5/>,  spores;  col,  columella;/,  foot;  ps,  pseu- 
dopodium. 


enlarges  the  wall  of  the  archegonium  is  ruptured  and  the 
top  portion  of  it  is  carried  up  as  a  cap  on  the  spore-case, 
forming  the  calyptra.  The  lower  cells  produced  by  the 
dividing  oosperm  become  organized  into  a  much  swollen 


LIFE  HISTORY   OF  A  MOSS  20I 

foot,  imbedded  in  the  tissues  below,  and  connected  with 
the  spore-case  by  a  very  short  stalk  (Figs.  143  and  145). 

184.  The  Sporoph3rte. — It  will  have  been  recognized 
already  that  the  simple  structure  just  described,  since  it 
bears  spores,  is  the  sporophyte  stage  of  Sphagnum} 
While  the  sporophyte  is  maturing,  the  apex  of  the  female 
branch  elongates,  forming  a  leafless  stalk,  a  half  inch  or 
more  in  length.  This  stalk  is  called  the  pseudo podium 
(false  foot).  The  development  of  the  pseudopodium, 
coincident  with  that  of  the  sporophyte  is  very  interest- 
ing, and  the  question  at  once  naturally  arises,  as  to  how 
this  correlation  is  brought  about.  No  positive  explana- 
tion has  ever  been  given,  but  it  seems  probable  that,  as  the 
sporophyte  begins  to  develop,  the  cells  of  the  foot  excrete 
some  substance  which  stimulates  the  cells  in  which  it  is 
imbedded  to  divide  and  enlarge,  resulting  finally  in  the 
formation  of  the  pseudopodium.  The  advantage  of  the 
pseudopodium  in  facilitating  the  distribution  of  spores 
by  raising  the  spore-case  higher  into  the  air  and  well 
above  the  perichaetial  and  other  leaves,  is  obvious. 

The  sporophyte  of  Sphagnum  possesses  no  chlorophyll, 
and  consequently  does  not  elaborate  any  food,  obtaining 
its  entire  supply  from  the  sphagnum-plant  by  absorption 
through  the  foot.  Numerous  groups  of  stomatal  guard- 
cells  occur  on  the  wall  of  the  spore-case,  but  they  have  no 
slit  between  them — no  true  stomata — and  are  there- 
fore functionless.  There  are  also,  underneath  the  guard- 
cells,  no  intercellular  spaces,  such  as  are  always  associated 
with  true  stomata.  The  presence  of  these  functionless 
stomata  is  thought  by  some  botanists  to  indicate  that  the 

^  Such  simply  organized  sporophytes  are  commonly  called  sporogonia 
(singular  sporogonium). 


202 


STRUCTURE   AND    LIFE    HISTORIES 


sporophytes  of  the  ancestors  of  Sphagnum  possessed  true 
stomata  and  the  function  of  photosynthesis. 

185.  Formation  of  Spores. — As  the  spore-case  develops, 
the  inner  cells  become  differentiated  into  two  kinds,  one 
composing  the  larger  part  of  the  tissue,  and  the  other, 
larger  and  richer  in  protoplasm,  forming  a  dome  of  sporo- 

genous  or  spore-forming  tissue 
near  the  upper  wall  (Fig.  145). 
From  this  tissue,  spore- 
mother-cells  are  developed, 
and  from  each  of  these,  by 
reducing  divisions,  as  in  the 
fern,  four  spores. 

186.  Asexual  Reproduc- 
tion.— While  the  spores  are 
maturing,  a  circular  groove 
{annulus)  is  formed  near  the 
apex  of  the  spore-case,  and 
the  cells  in  this  zone  have 
thinner  walls  than  those  ad- 
jacent (Fig.  145).  At  the 
maturity  of  the  spore-case  these  cells  become  dry,  and  are 
easily  torn  apart,  thus  forming  a  lid,  or  operculum^  at  the 
summit  of  the  spore-case.  The  falling  away  of  the 
operculum  affords  an  opportunity  for  the  scattering  of  the 
spores.  Under  favorable  conditions  the  spores  germinate, 
putting  forth  a  very  short,  green  protonema,  as  in  the  case 
of  fern-spores.  The  tip  of  the  protonema  soon  broadens 
out,  forming  a  prothallus,  much  like  that  of  the  fern  in 
shape,  but  being  only  one  cell  thick  (Fig.  146).  Rhizoids 
form  on  the  under  side,  and  from  the  margin  other 
threads  develop,  having  chlorophyll,  and  resembling  the 


Fig.  146. — Spliagnumsp.  A,B, 
young  protonemata;  C,  older  pro- 
tonema with  leafy  bud,  k;  r,  mar- 
ginal rhizoids.     (After  Campbell.) 


LIFE   HISTORY   OF   A   MOSS  203 

protonema.  At  the  tips  of  each  of  these  threads  a  thallus 
may  also  form,  and  in  this  way  vegetative  multiplication 
is  brought  about.  From  each  thallus  arises  an  upright 
leafy  branch — the  sphagnum-plant  described  above.  The 
reader  has  already  recognized  that  this  complex  phase  of 
sphagnum  is  the  gametophyte. 

187.  Life-cycle  of  Sphagnxim. — The  life-cycle  of  sphag- 
num may  be  summarized  as  follows: 

OUTLINE  OF  LIFE  HISTORY  OF  SPHAGNUM 
Sphagnum-plant   {gametophyte) 

— i ^ 

T  T 

Anthericlial  branch  Archegonial  branch 

i  4- 

Antheridia  Archegonia 

4.  I 

Sperm  (male  gamete)  Egg  (female  gamete) 

Fertilization 


Oosperm     (zygote) 

ii 

Embrvo 

a 

Mature  Sporophyte  (mature  zygote) 

ii 

Sporangium 

ii 

S  pore-mother-cell 


Reduction 


1  1  1  1 

Spore  Spore  Spore   Spore 

i 

Protonema 

4- 

Thallus 

i 

Sphagnum-plant  {gametophyte) 

188.  Diagram  of  Life-cycle. — The  life-cycle  of  Sphag- 
num may  be  diagrammatically  represented  as  follows: 


204 


STRUCTURE    AND    LIFE    HISTORIES 


Fig.  147. — Diagram  of  life-cycle  of  sphagnum. 


189.  Other  Mosses. — The  so-called  ''true  mosses," 
with  which  we  are  perhaps  more  familar  than  with 
sphagnum,  cannot  be  studied  here  in  detail,  but  it  may  be 
said  that,  in  broad  outline,  their  life  histories  are  closely 
similar  to  that  of  sphagnum.  The  protonema  does 
not  produce  a  thallus,  but  the  leafy  branches,  or  moss- 
plants,  arise  directly  from  the  filamentous  protonema 
(Fig.  148).  They  are  both  monoecious  and  dioecious.  In 
the  "true  mosses"  no  pseudopodium  is  formed,  but  the 
stalk  of  the  sporophyte  (so  very  short  in  Sphagnum)^ 
elongates  to  form  a  seta,  often  over  i  inch  in  length.  In 
the  spore-case,  or  capsule,  there  is  much  less  sporogenous 
or  fertile  tissue,  in  proportion  to  sterile  tissue,  than  in 
Sphagnum.  Moreover,  at  the  base  of  the  capsules,  in 
the  true  mosses,  occur  functional  stomata,  opening  into 
intercellular    spaces,    and    surrounded    by    chlorophyll- 


LIFE  HISTORY  OF  A  MOSS 


205 


bearing  cells.  Thus  photosynthesis  may  be  carried  on, 
though  of  course  to  a  very  limited  degree.  The  sporo- 
phyte  of  the  true  mosses  seems  to  occupy  an  intermediate 
position  between  those  of  Sphagnum  and  the  fern,  and,  as 
we  ascend  from  the  lower  form  in  Sphagnum  to  the  higher 
form  in  the  fern,  the  transition  is  largely  characterised  by  a 
decrease  in  the  amount  of  fertile  tissue  and  an  increase  in 
the  relative  amount  of  sterile  tissue  of  the  sporophytes. 


Fig.  148. — Protonemata  of  a  moss  bearing  young  gametophyte  buds. 

190.  Vegetative  Multiplication. — Extensive  experiments 
^seem  to  indicate  that  every  living  cell  of  a  moss-plant  can 
develop  protonemata — or  in  other  words  is  a  potential 
spore.  These  protonemata  like  those  produced  by  the 
germination  of  spores,  produce  buds  which  may  develop 
into  mature  plants.  The  production  of  entire  plants  or 
of  parts  of  plants  in  this  way,  by  portions  of  the  vegetative 
body;  is  called  regeneration.    In  some  species  of  mosses  the 


2o6 


STRUCTURE    AND    LIFE    HISTORIES 


Fig.  149. — Hair-cap  moss  {Polytrichum  commune).  A,  male  plant;  B, 
same,  proliferating;  C,  female  plant,  bearing  sporogonium;  D,  same;  g, 
gametophyte;  s,  seta;  c,  capsule;  0,  operculum;  a,  calyptra,  E,  top  view  of 
male  plant. 


207 


Fig.  150.-A  moss  (T^^/raM^'^^^^^),  showing  gemmae;  G,  a  gemma  enlarged. 
(Cf.  Fig.  151.) 


^'°*  ^5;.— Photomicrograph  of  a  longitudinal  section  of 
{.letraphts),  showing  gemmaj  (g).     (Cf.  Fig.  150.) 


2o8  STRUCTURE   AND    LIFE    HISTORIES 

leafy-shoot,  and  in  others  the  protonemata,  may  give  rise 
to  special  small  bodies,  called  genimcB,  which  may  become 
separated  from  the  parent  plant  and  give  rise  to  new  plants 
(Figs.  150  and  151).  Gemmae  will  be  illustrated  more 
fully  in  the  liverworts  to  be  discussed  in  the  next  chapter. 
191.  Comparison  with  the  Fern. — By  comparing  Sphag- 
num with  a  fern  several  points  of  interest  are  brought 
out.  In  the  first  place,  we  learn  that,  while  the  "fern- 
plant"  with  which  we  are  familiar  is  a  sporophyte,  the 
sphagnum-plant  is  a  gametophyte.  In  the  second  place, 
while  the  sporophyte  of  the  fern  is  at  first  dependent  on 
the  gametophyte  for  its  nutrition,  the  sporophyte  soon 
becomes  entirely  independent,  and  the  simply  organized 
gametophyte  perishes;  while  in  sphagnum  the  sporo- 
phyte is  the  much  more  simply  organized,  and  is  depend- 
ent upon  the  gametophyte  for  nutrition  throughout  its 
entire  life.  In  their  mode  of  reproduction,  however,  the 
two  plants  are  very  similar,  each  producing  haploid 
gametes  of  two  sexes,  male  and  female,  that  need  to  fuse 
in  fertilization;  the  product  of  fertilization  (zygote)  being 
diploid,  and  producing  a  spore-bearing  phase;  and  the 
spores,  haploid  again,  through  reduction,  giving  rise, 
without  nuclear  and  cell-fusion  to  the  haploid  gameto- 
phyte. In  each  case,  in  the  life-cycle,  gametophyte 
alternates  with  sporophyte,  fertilization  with  reduction, 
gametes  with  spores,  haploid  cells  with  diploid.  What 
takes  place  in  the  cells  between  fertilization  and  reduc- 
tion, and  between  reduction  and  fertilization?  This  is  one 
of  the  many  fascinating  problems  in  botany  still  awaiting 
solution.  There  is  only  one  way  by  which  the  answers 
to  these  problems  may  be  ascertained;  namely,  by  accu- 
rate, persistent,  painstaking  observation  and  experiment. 


CHAPTER  XVI 

LIFE  HISTORY  OF  A  LIVERWORT 

192.  Habitat. — The  group  of  plants  ranking  next  below 
the  mosses  in  the  scale  of  Hfe  is  the  liverworts  (Hepaticse). 
They  are  widely  distributed  over  the  earth's  surface; 
being  found  in  a  wide  climatic  range,  but  usually  in  moist 
situations.  Some  forms  (e.g.,  Riccia  natans)  may  grow 
floating  on  the  surface  of  water,  others  {e.g.,  Riccia fluitans) 


Fig.  152. — Anlhoceros  fusiformis.     Portion  of  lamellate,  cristate  thallus, 
which  easily  retains  water.     (After  M.  A.  Howe.) 

submerged;  but,  as  in  mosses,  no  salt-water  forms  have 
been  found.  A  few  species  grow  on  other  plants  {epi- 
phytic), or  in  other  situations  where  the  water  supply 
may  at  times  fall  very  low.  Such  forms  have  various 
contrivances  which  serve  to  retain  water.  Thus,  some 
species  of  Anthoceros  {e.g.,  A.  fusiformis,  A.  fimhriatus) 
possess  crisped  lobes,  forming  a  fringe  around  the  margin 
which  helps  the  plant  to  retain  water  (Figs.  152  and 
153).     In  another  species  {A.  punctatus),  water  is  retained 


14 


209 


2IO 


STRUCTURE    AND    LIFE    HISTORIES 


in  little  pits  or  depressions  on  the  upper  surface.  The 
habitat  of  the  Hverworts  illustrates  a  step  forward  in  the 
abandonment  by  plants  of  a  wholly  aquatic  Hfe  and  the 
establishment  of  a  land  vegetation;  but  the  prevailingly 
moist  situations  in  which  most  of  the  species  are  found, 


^l^M 

^, 

^■ 

^; 

,fe 

t           --. 
\ 

i 
"  '\ 

Fig.  153. — Anthoceros  fimhriatus.  Portion  of  a  thallus  viewed  from 
below,  with  the  rhizoids  omitted.  The  one-layered  crisped  lobes  at  the 
margin  serve  to  retain  moisture.     (After  Goebel.) 

and  the  need  of  water  for  fertilization  by  swimming 
sperms  (soon  to  be  described),  points  to  an  ancestral 
habitat  truly  aquatic. 

193.  Description  of  the  Plant  Body. — The  plant  body 
of  the  liverworts  shows  a  marked  departure  from  that  of 
the  mosses  in  the  direction  of  simplicity.  There  are  four 
main  groups  or  orders  of  Hepaticae,  as  follows: 

I  I.  Ricciales. 

TT       4.'      J  2.  Marchantiales. 
Hepaticae  \ 

3.  Jungermanmales 

.  4.  Anthocerotales. 


i 


LIFE  HISTORY  OF  A   LIVERWORT 


211 


In  the  first  two  and  last  of  these  orders  the  plant  body  is  a 
thallus,  either  closely  resembling  the  prothallus  of  the 
ferns,  or  freely  branching.     In  the  third  order  the  plant 


Fig.  IS4.-A  leafy  liverwort  {Porella  navicularis) .     Male  plant,  about 
natural  size.     (After  M.  A.  Howe.)  ^       ^       ^  ^ 

body  is  leafy  (Joliose),  and  in  this  respect  somewhat  re- 
sembles certain  true  mosses,  for  which  it  is  often  mis- 
taken (Figs.  154  and  155).  In  the  first  two  and  last  orders 
the  thallus  always  shows  dorso-ventral  differentiation. 


212  STRUCTURE    AND    LIFE    HISTORIES 

ANTHOCEROS 

194.  The  Gametophyte. — One  of  the  most  important 
groups  is  the  genus  Anthoceros^  including  several  different 
kinds  or  species.  The  plant  body,  or  thallus  (Fig.  156),  is 
roughly  circular  or  semicircular,,  with  numerous  rhizoids 
growing  from  the  ventral  surface.  It  increases  in  size  at 
numerous  growing  points  on  the  margin  of  the  thallus,  and 


Fig.  155. — A  leafy  liverwort  (Porella  navicularis) .     Female  plant,  about 
natural  size.     (After  M.  A.  Howe.) 

is  green  from  the  presence  of  chlorophyll  in  the  cells. 
There  is  only  one  chloroplast  in  a  cell,  in  contrast  to  the 
numerous  chloroplasts  in  each  cell  of  the  mosses  and  ferns. 
195.  Reproductive  Organs. — The  antheridia  are  found 
just  back  of  the  growing  points,  near  the  middle  of  the 
lobe.  In  some  species  they  occur  in  groups  (Figs.  157 
and  1 59),  in  other  species  singly  (Fig.  158) .     They  develop^ 


LIFE  HISTORY   OF  A  LIVERWORT 


213 


not  from  epidermal  cells  as  in  ferns,  but  each  one  from  a 
single  cell  just  underneath  the  epidermis  (subepidermal), 


Fig.  156. — Anthoceros  IcBvis,  showing  the  lobed  thallus  of  the  gameto- 
phyte,  bearing  several  upright  sporogonia  in  various  stages  of  develop- 
ment. At  the  right  and  left  sporogonia  dehiscing,  and  scattering  the 
spores.  Note  the  slender,  thread-like  columellas,  and  the  lack  of  differ- 
entiation of  the  sporogonium  into  seta  and  capsule.  The  sheoth  (calyp- 
tra)  at  the  base  of  the  sporogonium  is  formed  chiefly  from  the  vegetative 
tissues  of  the  gametophyte,  and  only  to  a  slight  extent  by  the  walls  of  the 
archegonium. 


Fig.  157. — Anthoceros  Pearsont.  Longitudinal  section  through  a  well- 
developed,  glandular  thickening,  in  which  are  embedded  a  number  of 
antheridia,     X  53.     (After  M.  A.  Howe.) 

near  the  dorsal  surface.     They  remain  imbedded  in  the 
surrounding  tissue  until  mature,   closely  resembling,  in 


214 


STRUCTURE    AND    LIFE   HISTORIES 


their  location  and  mode  of  origin,  the  antheridia  of  some 
of  the  lower,  or  eusporangiate,  ferns,  such  as  Ophioglosum 
and  BotrycJiium.  The  archegonia  are  also  imbedded,  with 
the  tip  of  the  neck  reaching  to  the  surface  (Fig.  159). 
They  are  further  concealed  at  maturity  by  the  growth  of 


Fig.  158. — Cross-section  of  the  thallus  of  a  horn  wort  (Anthoceras  sp.). 
The  oval  area  is  an  antheridium,  containing  sperms,  or  sperm-mother- 
cells. 


Fig.  159. — Anthoceros  fusiformis.  Vertical  longitudinal  section  near 
the  apex  of  the  thallus,  showing  archegonia  (at  the  left),  and  antheridia 
(at  the  right).     X  about  53.     (After  M.  A.  Howe.) 


a  small  dome  of  tissue  over  the  opening  where  the  neck 
comes  to  the  surface.  Both  antheridia  and  archegonia 
occur  on  the  same  plant,  sometimes  closely  intermingled. 
196.  Symbiosis. — A  most  interesting  case  of  symbiosis 
occurs  between  Anthoceros  and  a  much  more  lowly  organ- 


LIFE  HISTORY  OF  A  LIVERWORT 


215 


ized  plant — a  blue-green  alga,  of  the  genus  Nostoc  (Fig.  160). 
On  the  ventral  surface  of  the  thallus  slits  occur  in  the 
epidermis.  These  are  not  stomata,  and  the  intercellular 
spaces  into  which  they  open  are  fitted  with  a  mucilaginous 
substance  produced  by  a  transformation  of  the  adjacent 
cell-walls.  This  mucilage  furnishes  ideal  conditions  of 
food  and  moisture  for  the  alga,  which  flourishes  there. 


Fig.  160. — Photomicrograph  of  a  cross-section  of  a  liverwort  {Antho- 
ceros  fusiformis).  The  dark,  oval  area  is  a  colony  of  a  species  of  Nostoc, 
an  alga  that  lives  symbiotically  in  the  tissues  of  the  liverwort.  (Micro- 
scopic preparation  by  M.  A.  Howe.) 

Whether  the  presence  of  the  alga  is  of  any  advantage  to 
the  liverwort  is  not  known,  but  apparently  it  is  of  no 
disadvantage. 

197.  Vegetative  Multiplication. — ^Liverworts  present 
many  interesting  devices  for  vegetative  multiplication  by 
the  giving  off  or  separation  of  a  portion  of  the  vegetative 
tissue,  and  the  establishment  of  this  separated  piece  as  an 
independent  plant.  No  group  of  plants  excels  the 
liverworts  in  their  power  to  regenerate  new  individuals 
from  pieces  of  the  plant  body.     If  the  thallus  is  cut  into 


2l6 


STRUCTURE    AND    LIFE    HISTORIES 


many  small  pieces  with  a  pair  of  scissors,  each  piece  can 
regenerate  a  new  plant.  In  many  species  the  tips  of  the 
lobes  of  the  thallus  become  separated  from  the  plant 
naturally,  by  the  dying  off  of  portions  back  from  the  tip. 
In  such  cases  each  tip  develops  an  entire  new  individual. 
A  thorough  study  of  these  phenomena  has  led  botanists 


Fig.  i6i. — A  liverwort  {Lunularia).  Below,  portions  of  the  thallus, 
showing  the  lunar-shaped  cupules,  with  brood-buds,  or  gemmae.  Above 
a  single  gemma,  greatly  magnified. 


to  the  conclusion  that  every  cell  of  a  liverwort  is  able  to 
reproduce  an  entire  plant,  just  as  effectually  as  though 
it  were  a  spore.  Some  species  produce  little  multi- 
cellular bodies  called  gemmcs  (Fig.  i6i).  Other  species 
produce  fleshy  tubers,  richly  stored  with  reserve  food- 
materials,^  and  specially  valuable  in  helping  the  species 

^  Analogous  to  tuber-formation  in  the  potato. 


LIFE  HISTORY  OF  A  LIVERWORT 


217 


to  tide  over  periods  of  drought.  Tubers  in  liverworts  were 
first  discovered  and  recognized  in  a  species  of  Anthoceros. 
At  least  two  species  {Fossomhronia  tuherifera  and  An- 
thoceros tuberosus)  received  their  specific  names  from  their 


Fig.  162. — Anthoceros  phymatodes.     Portion  of  thallus  showing  develop- 
ing tubers.     X  about  15.     (After  M.  A.  Howe.) 

characteristic  of  forming  tubers.  In  some  species  the 
tubers  appear  as  swellings  or  outgrowths  on  the  under- 
side of  the  thallus;  in  others  (Figs.  162  and  163)  as  en- 
largements of  the  tips  of  thallus-lobes.  The  leafy  liver- 
wort, Bryopteris  filicina  (Fig.  164),  illustrates  vegetative 
multiplication  by  stolons. 


Fig.  163. — Anthoceros  phymatodes.     Mature  tuber,  sprouting.     X  about 
21.     (After  M.  A.  Howe.) 

198.  The  Sporophyte. — After  fertilization  the  oosperm 
develops  a  young  embryo,  and  from  the  lower  or  basal 
half  the  foot  develops,  with  projections  reaching  down 
into  the  tissue  of  the  gametophyte  (Fig.  165).     After  the 


2l8 


STRUCTURE    AND    LIFE   HISTORIES 


1>^^ 


early  stages  of  development  the  sporophyte  ceases  to  grow 
at  the  apex,  and  elongates  only  by  the  formation  and 
enlargement  of  new  cells  just  above  the  foot  {intercalary 
growth).  Around  the  base  of  the  sporophyte  there 
develops  from  the  tissue  of  the  gametophyte  a  sheath, 
but,  in  contrast  with  the  mosses,  no  seta  is  formed.     The 

appearance  of  the  sporophytes, 
as  they  appear  in  clusters,  has 
been  aptly  likened  to  tufts  of 
delicate  blades  of  grass  (Fig. 
156).  Spores  are  formed  from 
the  cylindrical  archesporium, 
between  outer  and  inner  layers 
of  sterile  tissue.  The  inner 
thread-like  layer  is  the  colu- 
mella (Fig.  156).  Chlorophyll 
develops  in  the  sterile  cells,  and 
intercellular  spaces  open  to  the 
surface  through  true  stomata 
(Fig.  166).  The  sporophyte, 
therefore,  has  the  function  of 
photosynthesis,  and  if  only  a 
root,  or  roots,  would  develop 
from  the  basal  portion,  it  could 
well  become  established  as  an 
independent  plant.  As  it  is,  it  can  live  only  as  long  as 
the  gametophyte  remains  active,  so  as  to  maintain  the 
supply  of  water  to  the  sporophyte.  The  columella  serves 
to  conduct  water  up  the  sporophyte. 

199.  Formation  of  Spores. — As  in  the  mosses  and 
ferns,  spores  arise  from  spore-mother-cells  by  reducing 
divisions     (tetrad-formation).     In    Anthoceros    they    do 


Fig.  164. — Bryopteris  fili- 
cina,  showing  long  shoots  and 
short  shoots.  The  stolons  at 
the  base,  with  reduced  leaves, 
serve  not  only  in  vegetative 
propagation,  but  also  to  anchor 
the  plant.     (After  Goebel.) 


LIFE  HISTORY  OF  A  LIVERWORT 


219 


not  all  mature  at  the  same  time,  as  in  the  mosses  and 
ferns,  but  new  spores  continue  to  form  in  the  region  of 
intercalary  growth  so  long  as  growth  continues.  As  the 
spores  mature  the  tip  of  the  sporophyte  splits  open,  and 
the  walls  spread  apart  progressively,  as  spores  lower  down 
come  to  maturity. 


i  '      i  " 

i 

^  1 

I'.,          'i-' 

ra  ffl^ 

^IKl". 

ii 

Fig.  165. — Anthoceros.  Photomicrograph  of  a  longitudinal  section  of 
the  sporogonium,  and  portion  of  the  gametophytic  thallus.  Note  the 
foot  of  the  sporogonium,  and  the  more  darkly  stained  spores  at  the  center, 
above. 


200.  Distribution  of  Spores. — In  addition  to  spores,  a 
portion  of  the  tissue  of  the  archesporium  develops  sterile 
cells  or  cell-rows,  called  elaiers  (Fig.  166).  When  these 
become  dry,  as  the  sporogonium  splits  open  from  the  tip, 


220 


STRUCTURE   AND   LIFE   HISTORIES 


Fig.  i66. — Anthoceros.     At  the  left,  a  stoma  with  guard-cells, 
right,  spores  and  pseudo-elaters. 


At  the 


Fig.  167. — Diagram  of  the  life-cycle  of  Anthoceros. 


LIFE  HISTORY  OF  A  LIVERWOtlT  221 

they  twist  with  a  jerking  motion,  which  helps  to  project 
the  spores  out  to  a  distance  from  the  parent  plant.  The 
advantage  of  this  is  self-evident. 

201.  Germination.— In  some,  if  not  all,  species  of 
Anthoceros,  the  spores  germinate  best  after  a  resting 
period  of  several  weeks  or  months.  Early  in  germination 
chlorophyll  develops  in  a  plastid  contained  in  the  spore, 
and  a  germination-tube,  or  protonema,  forms,  long  in 
some  species,  shorter  in  others,  and  at  the  tip  of  this  tube 
a  thallus  develops. 

202.  Summary  of  Life  History. — The  life  history  of 
Anthoceros  is,  in  outline,  as  follows  (it  is  illustrated  dia- 
grammatically  in  Fig.  167): 

OUTLINE  OF  LIFE  HISTORY  OF  ANTHOCEROS 

Anthoceros-plant    {garnet ophyte) . 

i 
—I  T^ 

Antheridia  Archegonia 

4.  i 

Sperms  Egg 

Fertilization 

Oosperm  (zygote) 

I  I 

Embryo 

4-  4- 

Mature  sporophyte  (mature  zygote) 

4-      Ar 

Spore-mother-cell 

X.  4,  4.  4,       1  Reduction 

Spore     Spore  Spore    Spore    J 

\r 

Protonema 

4. 

Anthoceros-plant  (gametophyte) 


2  22  STRUCTURE    AND    LIFE    HISTORIES 

OTHER   FORMS 

203.  Riccia. — About  4,000  species  of  liverworts  have 
been  described,  and  it  is,  of  course,  possible  here  to  refer 
to  only  a  very  few  of  the  forms,  chosen  because  they 
illustrate  some  special  idea  or  step  in  the  evolutionary 
development  of  plants.  In  addition  to  the  forms  already 
mentioned,  attention  should  be  called  to  the  genus  Riccia, 


Fig.  168. — A  IWtrwoTt  {Riccia  trichocar pa),  X  about  35.  Cross-section 
of  the  thallus,  showing  young  sporogonium  in  the  enlarged  venter  of  the 
archegonium.     (After  M.  A.  Howe.) 

which  is  of  interest  because  of  its  aquatic  mode  of  life, 
and  also  because  of  its  extremely  simple  sporophyte — the 
simplest  sporophyte,  in  fact,  of  all  plants  that  possess 
archegonia  (Archegonates) .  The  fertilized  egg  of  Riccia 
develops  a  sporophyte  which  has  only  fertile  cells  (spores), 
except  for  a  wall,  one  cell  thick,  enclosing  the  spores 
(Fig.  168).  In  fact,  the  sporophyte  consists  of  only  a 
very  simple  spore-case,  of  short  duration;  it  never  pro- 
jects beyond  the  venter  of  the  archegonium.  Spores 
are  formed  in  the  usual  way,  as  described  for  the  forms 


LIFE  HISTORY   OF  A  LIVERWORT 


^n 


previously  studied.     Except  for  the  sterile  wall-cells,  all 
the  cells  formed  by  the  successive  divisions  of  the  oopserm 


^h^M^f^ 


Fig.  i69.~Ricciocarpus.     sp.,  sporogonium;  gam,  tissue  of  gametophyte. 


Fig.  170.— Life  cycle  of  Ricciocarpus.     (After  Cardiflf.) 


become  spore-mother-cells,  each  of  which,  by  the  tetrad- 
division,  gives  rise  to  four  spores.     Meanwhile  the  wall- 


224  STRUCTURE    AND    LIFE    HISTORIES 

cells  become  disorganized  and  their  substance  goes  to  nour- 
ish the  spores.  By  this  time  the  cells  of  the  inner  layer 
of  the  venter  have  also  become  disorganized  and  their  sub- 
stance, in  like  manner,  is  absorbed  by  the  spores  (Fig.  169). 
Thus  the  tissue  of  the  gametophyte  serves  as  nourish- 
ment for  the  developing,  primitive  sporophyte.  The  phe- 
nomenon of  an  embryo  developing  into  a  mature  sporophyte 
does  not  appear  in  Riccia.  As  we  descend  the  scale  of  plant 
life  we  find  the  sporophyte  increasingly  simpler;  and  this 
simplicity  consists  in  the  diminution  of  the  amount  of 
sterile  tissue. 

204.  Life  History  of  Riccia. — The  life  history  of  Riccia 
is  shown  in  outline  as  follows,  and  diagrammatically  in 
Fig.  170.  Carefully  compare  this  history  with  that  of 
Anthoceros. 

OUTLINE  OF  LIFE  HISTORY  OF  RICCU 
Riccia-plant  {gametophyte) 


Oosperm  (zygote) 

Cell-divisions 

Sporophyte  (transient  spore-case) 

44- 

S  pore-mother-cclls 

\r  X^  X-  X     ]    Reduction 

Spore     Spore      Spore      Spore 

i 

Protonema 

i 

Riccia-plant  {gametophyte) 


LIFE    HISTORY    OF    A    LIVERWORT  225 

205.  Comparison  with  Mosses  and  Ferns. — {a)   The 

Gametophyte. — The  gametophytes  of  the  various  kinds  of 
liverworts  differ  greatly  among  themselves,  but  on  the 
whole  they  are  more  simply  organized  than  those  of  the 
mosses,  lacking  especially  the  highly  developed,  leafy 
branches  or  gametophores.  The  moss-plant  represents 
the  highest  degree  of  gametophytic  organization  known 
among  land-plants,  and  the  leafy  branch  is  practically 
universal  in  that  group.  On  the  other  hand,  the  vege- 
tative body  of  the  liverworts  is,  in  some  forms,  simpler 
than  the  prothallus  of  the  fern,  while  in  other  forms  it  is 
much  more  complicated,  becoming 'a  leafy  branch  in  the 
Jungermanniales,  and  bearing  complex  gametophores  and 
other  organs  in  the  Marchantiales.  But  while  it  may  be- 
come complex,  its  organization  is  always  of  a  lower  type 
than  that  of  the  moss-plant.  The  antheridia  are  much 
alike  in  both  mosses  and  liverworts,  and  on  the  whole 
differ  but  little  from  that  of  the  true  ferns;  but  the  sperma- 
tozoids  of  the  former  are  always  hiciliate,  while  those  of 
the  true  ferns  are  always  multiciliate.  The  archegonia 
of  mosses  and  liverworts  may  or  may  not  be  stalked,  but 
they  are  never  stalked  in  the  true  ferns.  With  the  ex- 
ception of  Anthoceros  they  are  never  sunk  beneath  the 
surface  in  either  mosses  or  liverworts,  bi:^t  in  the  ferns  the 
venter  is  commonly  sunk  in  the  tissue  of  the  prothallus. 

(h)  The  Sporophyte. — The  typical  sporogonium  or 
sporophyte  of  liverworts  and  mosses  consists  of  a  stalk 
or  seta,  with  a  foot  at  one  end,  imbedded  in  the  tissue 
of  the  gametophyte,  and  a  spore-case  at  the  opposite 
end.  There  are,  however,  all  degrees  of  variation  of  this 
type  of  structure.  The  stalk  and  foot  may  be  entirely 
wanting,  as  in  the  simple  sporophyte  of  Riccia;  the  stalk 

IS 


2  26  STRUCTURE    AND    LIFE   HISTORIES 

may  be  very  greatly  reduced,  as  in  Sphagnum;  or  the 
spore-case  may  not  appear  as  a  clearly  defined  organ, 
but  may  appear  to  merge  very  gradually  into  the  stalk,  as 
in  Anthoceros.  In  liverworts  the  spore-case  never  opens 
by  a  lid  or  operculum,  as  is  universally  the  case  in  mosses, 
but  always  by  valves,  formed  by  longitudinal  splittings 
of  the  sporangial  walls.  Elaters  may  or  may  not  occur  in 
liverworts,  but  never  occur  in  mosses.  The  sporogonium 
of  liverworts  and  mosses  never  possesses  a  leafy  stem,  and 
never  possesses  true  roots;  only  one  case  (that  of  the  moss, 
Eriopus  remotifolius)  has  ever  been  reported  where  the 
sporogonium  produces  rhizoids  from  its  basal  end.  To 
compare  the  simple  sporogonium  of  Hverworts  and  mosses 
with  the  leafy  plant  of  the  true  ferns,  would  be  quite  super- 
fluous. It  should,  however,  be  pointed  out  that  the  sporo- 
phyte  of  liverworts  and  mosses  lives  always,  throughout  its 
life,  as  a  parasite  on  the  gametophy te,  while  the  sporophy te 
of  ferns  always  becomes  established,  sooner  or  later,  as  an 
independent  plant.  Except  for  the  very  simple  columella 
of  Anthoceros  and  the  central  strand  in  the  seta  of  mosses, 
nothing  approaching  a  true  vascular  bundle  occurs  in  the 
liverworts  and  mosses;  while,  as  stated  before,  the  well- 
developed  fibro-vascular  system  of  the  ferns  has  caused 
them  to  be  known  as  vascular  cryptogams. 


CHAPTER  XVII 

LIFE  HISTORIES  OF  ALG^E 

206.  The  Main  Groups  of  Algae. — The  plants  that  rank 
next  in  the  scale  of  life  below  the  liverworts  are  the  Algae. 
On  the  basis  of  color  they  fall  naturally  into  four  main 
groups  or  phyla,  as  follows: 

1.  Blue-green  algae  (Cyanophyceae) 

2.  Green  algse  (Chlorophyceae) 

3.  Brown  algae  (Phasophyceae) 

4.  Red  algae  (Rhodophyceae) 

Associated  with  these  differences  in  color  are  certain  dif- 
ferences of  structure,  which  also  lead  to  a  similar  grouping. 
In  recent  years  the  above  four  groups  have  been  further 
subdivided  into  seven  phyla,  on  the  basis  of  other  char- 
acters than  color.  A  study  of  one  of  the  commoner  brown 
algae,  Ascophyllum,  will  serve  to  illustrate  many  funda- 
mental facts  about  the  algae  in  general. 

ASCOPHYLLUM 

207.  Habitat  of  Ascophyllimi. — This  plant,  and  the 
closely  related  Fucus,  have  become  familiar  in  inland 
markets,  because  they  are  commonly  used  as  a  packing  in 
the  shipment  of  crabs  and  other  kinds  of  "shell  fish." 
Ascophyllum  and  Fucus,  and  their  near  relatives,  consti- 
tute the  family  FucacecB.  For  the  most  part  they  are 
inhabitants  of  the  ocean,  or  of  brackish  water,  or,  in  rare 
cases,  of  fresh  water.     The  two  genera  mentioned  are 

227 


228 


STRUCTURE   AND   LIFE  HISTORIES 


LIFE    HISTORIES    OF   ALG^  229 

commonly  found  attached  to  rocks  between  the  lines  of 
high  and  low  tide,  where  they  are  subjected  to  alternate 
submersion  and  exposure  (Fig.  171).  The  Fucace^  have 
an  added  interest  because  of  their  economic  uses.  They 
serve  as  food  for  the  inhabitants  of  the  west  coast  of 
South  America,  and  in  other  countries,  and  are  also  widely 
used  as  fertilizer,  and  as  a  source  of  iodine.     They  include 


Fig.  172.— Portion  of  plant  of  Ascophyllufn  nodosum.  X  %. 


some  of  the  largest  plants  in  the  ocean,  and  one  of  the 
genera,  Sargassum,  floating  on  the  surface,  helps  to  form 
the  well-known  -  Sargasso  Sea,"  of  the  middle  Atlantic 
ocean. 

208.  Description  of  AscophyUum.— The  plant  body  of 
Ascophyllum  is  a  branched  thallus,  the  branches  being 


330 


STRUCTURE    AND    LIFE    HISTORIES 


in 


relatively  narrow  (from  J^  to  J-^  inch),  several  feet 
length,  and  interrupted  at  frequent  intervals  by  swellings 
or  nodes,  which  are  air  sacs,  and  add  greatly  to  the  buoy- 
ancy of  the  plant  in  water  (Fig.  172).  Many  of  the  Fucaceae 
possess  two  kinds  of  branches,  more  or  less  distinct  from  each 
other — long  branches  and  short  branches,  or  spurs.  This 
is  a  phenomenon  which  occurs  in  several  groups  of  plants. 


Fig.  173. — Ascophyllum  nodosum  (L)  Lejol.  Radical  longitudinal  sec- 
tion of  an  old  branch  of  the  thallus.  c,  cortical  tissue,  the  seat  of  photo- 
synthesis; m,  central  tissue,  or  medulla.     (Redrawn  from  Reinke.) 


and  notably  in  the  pines,  to  be  studied  later.  In  Ascophyll- 
um the  distinction  between  long  and  short  branches  is  not 
as  strongly  marked  as  in  some  other  forms,  such,  for  ex- 
ample, as  Scaheria.  The  short  branches  have  enlarged 
tips,  which  somewhat  resemble  the  swellings  of  the  main 
stem.  The  plant  has  a  "rubbery"  appearance,  with  a 
smooth,  slippery  surface,  and  is  usually  attached  to  rocks 
by  a  "hold-fast"  organ. 

209.  Anatomy. — A  study  of  the  internal  structure 
(Fig.  173)  reveals  two  systems  of  tissues,  more  or  less 
clearly  distinct: 


LITE   HISTORIES   OF   ALG.E  23 1 

1.  The  cortex,  composed  of  several  external  layers  of 
cells,  somewhat  resembling,  in  arrangement,  the  palisade 
parenchyma  of  the  leaf  and,  like  the  latter,  having  the 
function  of  photosynthesis.  The  outer  portion  of  this 
layer  is  further  loosely  differentiated  into  an  epidermoidal 
tissue,  but  there  is  no  true  epidermis.  The  outer  cell- 
walls  of  this  layer,  forming  the  external  surface  of  the 
plant,  possess  a  thick  layer  of  cuticle.  The  cells  of  the 
cortex  retain  their  embryonic  character  for  a  long  time, 
and  by  successive  divisions  favor  the  growth  of  the  branch 
in  thickness. 

2.  The  medulla,  or  central  tissue,  is  composed  of  cells 
arranged  for  the  most  part  in  rows,  so  as  to  form  filaments. 
This  tissue  serves  chiefly  for  the  conduction  of  liquids. 
The  walls  of  its  cells  are  mucilaginous  and  much  swollen, 
except  at  certain  small  pits,  the  "sieve  tubes,"  closed  by 
a  perforated  membrane. 

210.  Photosynthesis.— The  cells  of  the  cortex  possess 
several  chromoplasts  or  chromatophores,  each  containing 
chlorophyll,  by  which  photosynthesis  is  possible;  but,  in 
addition  to  chlorophyll,  the  chromatophores  contain  also 
a  brown  pigment  {phycophwin) ,  which  masks  the  chloro- 
phyll, and  explains  the  external  color  that  gives  the  name 
Phaeophyceae  to  the  family. 

211.  Vegetative  Multiplication. — Vegetative  multiplica- 
tion does  not  occur  in  Ascophyllum,  nor  in  most  of  the 
genera  of  Phaeophyceae.  In  the  few  genera  where  it  has 
been  observed,  it  is  accomplished  by  a  fragmentation  of  the 
plant  body,  or  by  the  death  of  the  older  part  of  the  thallus. 
The  pieces  thus  set  free  may  develop  new  plants,  but  these 
usually  remain  sterile. 


232 


STRUCTURE   AND   LIFE   HISTORIES 


212.  Sexual  Reproduction. — The  reproductive  organs 
of  Ascophyllum  (Fig.  174)  are  borne  in  chambers 
(conceptacles)  beneath  the  surface  of  the  enlargements  at 
She  tips  of  the  short  branches.     Since  the  branches  bear 


^.f^H'lS?-^. 


^^^^^l^? 


Fig.  174. — Ascophyllum  nodosum.  A,  Cross-section  through  a  female 
receptacle;  5,  spermagonia;  C,  ripe  oogonium;  D,  eggs,  freed  from  the 
oogonium,  but  still  enclosed  by  the  separated  inner  layer  of  the  oogonial 
wall.     (Redrawn  from  Thuret  and  Bornet.) 


the  gametes  they  are  sometimes  referred  to  as  gameto- 
phores.  These  chambers  open  to  the  exterior  by  short, 
narrow  canals,  the  openings  of  which  may  be  easily  seen 
on  the  surface  of  the  swollen  tips.     The  inner  surface  of 


LIFE   HISTORIES   OF   ALG^  233 

the  conceptacles  is  covered  with  more  or  less  branched 
hairs  (paraphyses) ,  and  associated  with  these  hairs  are  the 
organs  that  bear  the  sperms  and  eggs.  In  some  species 
of  Fucaceae  both  sperms  and  eggs  are  borne  in  the  same 
conceptacle,  and  the  plant  is,  accordingly,  moncscious. 
This  is  the  case  with  one  of  the  species  of  Fucus  {F, 
platycarpus).  In  other  species,  such  as  those  of  Asco- 
phyllum  and  Fucus  vesiculosus,  sperms  and  eggs  are  borne 
in  separate  conceptacles,  and  even  on  separate  plants,  in 
which  latter  case  the  species  are  dioecious. 

213.  Gametangia.— The  organs  that  bear  either  kind 
of  gametes  (sperms  or  eggs)  are  termed  gametangia.  The 
female  gametangium  differs  in  a  very  fundamental  manner 
from  the  complex  archegonium  of  the  mosses  and  ferns,  for 
it  consists  of  only  one  cell,  called  the  oogonium.  The 
male  gametangium,  or  spermagonium,  is  likewise  unicel- 
lular, and  the  wall  is  composed  of  two  layers,  an  inner  and 
an  outer  layer.  The  spermagonia  are  in  reaKty  modified 
branches  of  the  hairs  that  line  the  conceptacles.  The 
oogonia  are  not  attached  to  the  hairs,  but  directly  to  the 
inner  surface  of  the  conceptacle  by  a  short  unicellular 
stalk. 

214.  Gametes.— The  male  gametes,  or  sperms,  are 
formed  by  successive  divisions  of  the  protoplast  of  the 
unicellular  spermagonium.  Like  those  of  the  Hverworts 
and  mosses,  they  bear  two  long  ciHa,  attached  to  the  side. 
They  also  possess  a  pigment  body,  usually  reddish  in 
color.  The  oogonia  of  Ascophyllum  commonly  bear  only 
four  eggs,  organized  out  of  the  protoplast  of  the  oogonium, 
but  in  rare  cases  three  or  five.  In  some  genera  {e.g., 
Fucus)  there  are  eight  eggs.  The  nucleus  of  the  oogonium 
cell   usually  divides  into  eight  daughter-nuclei,  but  in 


234  STRUCTURE    AND    LIFE   HISTORIES 

Ascophyllum  all  except  four  (or  the  exceptional  three  or 
five)  fail  to  organize  daughter-cells  about  themselves,  and 
abort.  Forms  having  the  full  complement  of  eight  eggs, 
are,  therefore,  considered  more  primitive  than  those  with  a 
less  number. 

216.  Fertilization. — The  eggs  are  never  fertiHzed  while 
in  the  oogonia,  nor  even  while  in  the  conceptacle.  The 
walls  of  the  oogonium  burst,  and  the  eggs  pass  out  into  the 
surrounding  water.  They  are  covered  with  a  thick  layer 
of  mucilaginous  substance,  and  by  means  of  some  material, 
not  definitely  known,  they  attract  the  sperms  that  happen 
to  have  been  discharged  at  the  same  time  and  near  the 
same  place.  No  other  case  is  known  in  plants  where  the 
difference  in  size  between  egg  and  sperm  is  so  great  as  in 
the  Fucaceae  (Fig.  175).  The  sperms  swarm  about  an  egg, 
and  finally  one  of  them  enters  it  and  its  nucleus  unites 
with  that  of  the  egg,  thus  completing  fertilization.  Soon 
after  fertilization  the  oosperm  or  zygote  becomes  sur- 
rounded by  a  dehcate  cellulose  wall,  the  fertilization- 
membrane.  The  setting  free  of  the  egg  before  fertilization 
marks  a  lower  stage  of  development  than  is  found  in  the 
mosses  and  ferns. 

The  process  of  fertilization  in  Fucus  may  be  easily 
observed  by  placing  mature  eggs  and  sperms  together  in 
sea-water  in  a  watch  glass,  under  the  microscope.  The 
sperms,  attracted  by  the  chemical  stimulus  of  the  sub- 
stance excreted  by  the  egg,  swim  toward  it,  and  within 
about  five  minutes  large  numbers  of  them  have  become 
attached  to  its  surface.  By  the  vigorous  lashing  of  their 
ciha  the  egg  is  set  in  vigorous  motion.  One  of  the  sperms 
succeeds  in  penetrating  the  cytoplasm  of  the  egg,  and 
reaches  its  nucleus.     The  fusion  of  the  two  nuclei  may 


LIFE   HISTORIES   OE   ALG^E 


235 


occur  within  lo  minutes  after  the  gametes  have  been 
placed  together  in  the  water.     After  the  formation  of  the 


Fig.  175. — Ascophyllum  nodosum  (L.)  Lejol.  Ay  egg  at  the  time  of 
fertilization,  surrounded  by  numerous  sperms.  X  about  200;  B,  oosperm 
germinating,  6  days  old;  C,  D,  somewhat  older  stages  than  B.  (Redrawn 
from  Thuret  and  Bornet.) 


fertilization-membrane  the  sperms  seem  to  avoid  the 
oosperm,  quite  as  though  they  were  repelled  by  some 
substance  formed  at  its  surface. 


236  STRUCTURE    AND    LIFE    HISTORIES 

216.  The  Result  of  Fertilization.— (a)  The  immediate 
result  of  fertiUzation  is  physical— the  formation  of  the 
fertiHzation-membrane.  Just  how  this  is  accompHshed  is 
not  clearly  understood.  We  know,  of  course,  that  the 
surface  of  the  egg,  as  in  every  free  mass  of  protoplasm, 
acts  as  a  semipermeable  membrane  or  surface,  allowing 
some  substances  in  solution,  but  not  all,  to  pass  through 
by  osmosis.  It  has  been  suggested  that,  when  the  sperm 
enters  the  egg,  chemical  changes  at  once  occur,  which 
alter  the  permeability  of  its  surface-membrane,  thus  per- 
mitting, for  the  first  time,  the  exosmosis  of  some  substance 
(or  substances)  which  become  transformed  into  the  fertiliza- 
tion-membrane on  contact  with  the  sea-water.  It  may 
be,  of  course,  that  the  substance  composing  the  mem- 
brane is  not  formed  until  the  sperm  enters  the  egg.  How- 
ever it  may  be  caused,  the  formation  of  the  membrane  is 
a  necessary  antecedent  to  all  subsequent  changes  in  the 
fertilized  egg;  without  its  formation  the  egg  dies  and  dis- 
integrates. 

(b)  The  ultimate  result  of  fertilization,  as  noted  in  the 
preceding  chapters,  is  biological — the  intermingHng  of  the 
germ-plasms  of  the  egg  and  sperm,  involving  the  fusion 
of  the  two  nuclei,  the  doubling  of  the  chromosome  number, 
and  the  combination,  in  one  zygote,  of  the  inheritances 
from  two  individuals.^ 

217.  Artificial  Fertilization. — Considering  that  the  for- 
mation of  the  fertiHzation-membrane  is  purely  physical, 
biologists  began  to  reason  that  it  ought  to  be  possible  to 
induce  it  artificially.  The  experiment  was  first  success- 
fully made  by  a  zoologist,  Loeb,  with  the  eggs  of  sea-urchins 

^  The  commingling  of  the  two  inheritances  was  called,  by  Weismann, 
amphimixis. 


LIFE   HISTORIES    OF   ALGiE  237 

and  other  marine  animals.  In  19 13  it  was  successfully 
accomplished  by  Overton  with  the  eggs  of  Fucus.  The 
eggs  were  dipped  for  about  a  minute,  or  a  minute  and  a 
half  to  two  minutes,  in  a  mixture  of  50  cc.  of  sea-water 
plus  3  cc.  of  a  very  weak  solution  of  acetic,  butyric,  or 
other  fatty  acid,  and  then  transferred  to  normal  sea- 
water.  This  treatment  caused  the  formation  of  the  fertiH- 
zation-membrane,  quite  as  in  natural  fertilization  by  the 
sperm.  If,  after  the  formation  of  the  membrane,  the  eggs 
are  placed  for  30  minutes  in  hypertonic  sea-water  (50  cc. 
of  normal  sea-water  plus  8  to  10  cc.  of  a  weak  solution  of 
sodium  chloride  (common  salt),  or  potassium  chloride), 
and  then  back  into  normal  sea-water,  the  eggs  begin  to 
divide  and  continue  to  develop  into  young  plants.  The 
question  as  to  the  chromosome  number  in  the  cells  of 
plants  formed  by  artificial  fertilization  is  of  very  great 
interest,  but  has  not  yet  been  investigated. 

218.  Germination  of  the  Oosperm. — After  either  natural 
or  artificial  fertilization  the  young  zygote  begins  at  once 
to  divide,  without  any  period  of  rest.  Of  the  two  cells 
formed  by  the  first  division,  one  gives  rise  to  the  hold-fast 
organ,  by  which  the  new  plant  is  attached  to  the  rocks, 
while  the  other  develops  into  the  main  body  of  the  plant, 
which  resembles  the  parent  plant  in  all  external  characters 
(Fig.  175)- 

219.  Reduction. — As  always  in  normal  fertilization,  the 
nucleus  of  the  oosperm  is  diploid,  and  the  Ascophyllum 
plant  that  develops  from  it  is  also  diploid.  It  is  therefore 
the  sporophytic  generation.  At  the  end  of  the  first  two 
nuclear  divisions  of  the  spermagonia  and  oogonia,  re- 
duction has  been  accomplished,  and  the  four  nuclei  that 
result  are  haploid.     They  therefore  belong  to  the  haploid 


238  STRUCTURE    AND    LIFE    HISTORIES 

or  gametophytic  generation.  In  other  words  they  have 
the  same  value  as  spores,  and  the  one-celled  stage  of  the 
spermagonia  and  oogonia  the  same  value  as  spore-mother- 
cells. 

220.  Female  Gametophyte. — Each  of  the  four  cells  re- 
sulting from  the  reduction-divisions  in  the  oogonial 
protoplast  divides  again,  producing  a  total  of  eight  cells, 
which  constitute  a  very  simple  gametophyte.  No  further 
divisions  occur  in  the  oogonia.  In  some  of  the  Fucaceae 
{e.g.f  Fucus  vesiculosus)  each  of  these  eight  daughter-cells 
functions  as  a  female  gamete  or  egg;  but  in  Ascophyllum, 
and  a  few  other  species,  part  of  the  daughter-cells,  as 
stated  above,  disintegrate  or  abort,  leaving  only  from  one 
to  five.  In  Ascophyllum  nodosum  one-half  of  them  abort, 
leaving  only  four  eggs.  The  female  gametophyte  is  thus 
seen  to  be  reduced  to  merely  its  gametes. 

221.  Male  Gametophyte. — Each  of  the  four  cells 
resulting  from  the  reduction-divisions  of  the  spermagonial 
protoplast  undergoes  four  divisions  in  succession,  re- 
sulting in  64  cells  or  a  total  of  256,  all  of  which  develop 
into  a  male  gamete,  or  sperm.  The  four  daughter-cells, 
therefore  represent  a  very  elementary  or  simple  male 
gametophyte. 

222.  Simplification  of  the  Gametophytes. — The  im- 
portant point  to  note  in  connection  with  the  life  history  of 
Ascophyllum  is,  not  only  the  great  simplicity  of  the  game- 
tophyte, but  the  fact  that  it  consists  of  nothing  but 
fertile  or  reproductive  cells.  Each  of  the  four  spores 
gives  rise  only  to  gametes;  no  sterile  cells,  or  gametophytic 
plant  bodies  are  produced. 

223.  Gametophyte  or  Sporophyte.— In  light  of  the  facts 
above  related,  the  question  as  to  the  real  nature  of  the 


LIFE   HISTORIES    OF   ALGJE  239 

plant  body  of  Ascophyllum  becomes  of  very  great  interest. 
Its  cells  possess  the  double  or  sporophytic  number  of 
chromosomes,  but  it  bears  organs  (spermagonia  and 
oogonia)  that  ultimately  contain  gametes.  Is  it,  therefore, 
a  gametophyte  or  a  sporophyte?  For  a  long  time  it  was 
considered  a  gametophyte,  but  a  clear  understanding  of 
the  divisions  that  take  place  in  the  gametangia,  accom- 
panied by  reduction,  and  the  fact  that  the  body-cells  are 
all  diploid,  lead  unmistakably  to  the  conclusion  that  it  is 
a  sporophyte.  We  have  seen  that  the  protoplasts  of  the 
young  spermagonium  and  the  young  oogonium  are  in 
reality  equivalent  or  analogous  to  spore-mother-cells,  and 
that,  in  each  case,  their  four  daughter-cells,  with  their 
reduced  number  of  chromosomes,  are  functionally  equiva- 
lent or  analogous  to  spores.  The  spermagonia  and 
oogonia,  therefore,  which  seem  at  first  thought  to  be  sexual 
organs — simplified  antheridia  and  archegonia — come  to  be, 
finally,  more  truly  comparable  to  sporangia,  from  which 
the  spores  are  not  set  free,  as  spores,  but,  while  still  in  the 
spore-case,  develop  into  either  a  male  or  a  female  gameto- 
phyte consisting  of  nothing  but  gametes. 

The  real  nature  of  Ascophyllum  (and  of  the  other 
Fucaceae,  for  that  matter)  is  just  opposite  from  what  a 
superficial  examination  would  lead  us  to  infer,  and  we 
have,  in  this  low  form,  a  condition  just  the  reverse  from 
what  is  found  in  the  liverworts,  mosses,  and  ferns;  in  other 
words,  a  prominent  sporophyte,  bearing  a  very  simple 
gametophyte,  that  lives  upon  it  as  a  parasite  deriving  all 
of  its  nourishment  from  the  sporophyte. 

224.  Alternation  of  Generations. — Although  the  game- 
tophytic  generation  is  reduced  to  its  lowest  terms,  the 
fundamental  fact  of  alternation  is  not  affected.     As  soon 


240 


STRUCTURE   AND   LIFE   HISTORIES 


as  the  gametes  are  set  free  fertilization  takes  place,  pro- 
ducing a  fertilized  egg,  with,  of  course,  the  double  or  diploid 
number  of  chromosomes.  Every  cell  of  the  sporeling, 
resulting  from  the  germination  of  the  oosperm,  and  every 
cell  of  the  mature  plant  into  which  it  develops,  possesses 
the  double  number,  up  to  the  first  two  divisions  of  the 


■•■■^^  1  Stage 


Fig.  176. — Diagram  of  life-cycle  of  Ascophyllum  nodosum. 


young  gametangia,  when  reduction  gives  rise  to  gameto- 
phytic  cells.  The  fusion  of  these — egg  with  sperm — in 
fertilization  restores  the  double  or  sporophytic  number 
and  so  on  in  cycle  after  cycle  (Fig.  176). 

225.  Diagram  of  Life  History. — The  successive  steps  in 
the  life  cycle  of  Ascophyllum  may  be  briefly  summarized 
as  follows: 


LIFE    HISTORIES    OF   ALG^E  24I 

OUTLINE  OF  LIFE  HISTORY  OF  ASCOPHYLLUM 

Spermagonial-plant  Oogonial-plant 

(Sporophyte)  (Sporophyte) 

Spermagonium  Oogoniun 

(Sporangium)  (Sporangium) 

Protoplast  of  spermagonium  Protoplast  of  oogonium 

(spore-mother-cell)  (spore-mother-cell) 

4-  >l<  4^  4^     Reduction     '^  ^  ^  ^ 

Spore      Spore     Spore      Spore  Spore    Spore     Spore    Spore 

64  sperms  8  less  4  eggs 

(Reduced  male  gametophyte)  (Reduced  female  gametophyte) 


Oosperms    (rest) 


Spermagonial-plants     Oogonial-plants 
(Sporophytes)  (Sporophytes) 

226.  The  Cause  of  Sex. — One  of  the  many  very  inter- 
esting things  revealed  by  a  study  of  the  life  history  of 
Ascophyllum  nodosum  is  the  fact  that  part  of  the  fertilized 
eggs  give  rise  to  plants  that  bear  only  male  gametes,  and 
the  remainder  to  plants  that  bear  only  female  gametes. 
Since  the  fundamentally  different  individuals  develop 
under  identically  the  same  environmental  influences,  we 
are  forced  to  the  inference  that  the  difference  lies  in  the 
fertilized  eggs  themselves.  Some  of  them  appear  to  be 
male-producing,  others  female-producing.  In  what  does 
this  fundamental  difference  consist?  Here  is  a  very 
important  problem,  but  further  consideration  of  it  must 
be  postponed  until  Chapter  XXII. 


x6 


CHAPTER  XVIII 
LIFE  HISTORIES  OF  ALG^  (CONCLUDED) 
DICTYOTA  DICHOTOMA 

227.  Habitat. — ^Like  Ascophyllum  and  Fucus,  Dictyota 
dichotoma  grows  chiefly  along  the  ocean  margins  in  the 
zone  between  the  lines  of  high  and  low  tide.  It  is  thus 
subjected  to  alternate  wetting  and  drying,  and  to  rhyth- 
mically alternating  changes  of  light  and  temperature.     It 


m^^'^^'^iK^TTlH 

^Br\'  1  :-k  ,     '  i  ^ . '  ^jI^  ^\_  1  \  k  M^^^^^k 

B^S^ 

Fig.  177. — Dictyota  dichotoma.    Left,  sporogonial  plant;  right  sperma- 
gonial  (gametophytic)  plant.     (After  W.  D.  Hoyt.) 

is  found  from  as  far  north  as  the  middle  of  the  Scandi- 
navian peninsula  to  about  40°  south  latitude. 

228.  Description.— The   plant   body    (Fig.    177)    is   a 
thallus,  flat,  and  branching  profusely  by  forking  at  the 

242 


LIFE    HISTORIES    OF   ALGiK  243 

tips,  i.e.y  dichotomously  (whence  the  nsime  dicholoma) ,  but 
showing  no  differentiation  into  anything  like  stem  and 
leaf.  One  end  of  the  plant  is  differentiated  into  a  special 
branching  organ,  the  hold-fast,  by  which  it  becomes 
attached  to  rocks,  shells,  and  other  convenient  solids. 
As  in  Fucus,  again,  the  green  chlorophyll  is  masked  by  a 
brown  pigment,  which  indicates  the  relationship  of  the 
plant  to  the  Brown  seaweeds,  or  Phaeophyceae.  There  are 
two  kinds  of  branches — cylindrical  ones  which  are  sterile, 
and  others  more  strap-shaped,  or  ribbon-like,  which 
eventually  bear  the  reproductive  organs. 

Vegetative  multiplication  may  occur  by  the  separation 
of  portions  of  the  thallus,  which  may  become  established  as 
independent  plants.  In  some  species  of  Didyota,  specially 
differentiated  bodies  have  been  noted,  resembling  the 
brood-buds  or  gemmae,  such  as  occur  in  some  of  the  liver- 
worts and  mosses. 

229.  Reproduction. — With  reference  to  reproductive 
organs,  three  kinds  of  plants  occur : 

1.  Plants  bearing  asexual  spores  only  (asexual). 

2.  Plants  bearing  oogonia  only  (female). 

3.  Plants  bearing  antheridia  only  (male). 

But  the  most  interesting  feature  in  this  connection  is  that 
the  plants  of  all  three  groups  look  very  much  alike,  except,  of 
course,  for  their  different  kinds  of  reproductive  organs, 
and  for  unimportant  individual  variations. 

Oogonia  and  antheridia  are  both  produced  from  surface 
cells.  The  surface  cell  first  divides  into  a  smaller  stalk- 
cell,  and  a  larger  external  one,  rich  in  protoplasm,  which 
forms  the  gametes.  The  protoplast  of  the  oogonium  does 
not  divide,  as  in  Ascophyllum  and  Fucus,  but  develops 
into  only  one  very  large  egg;  while  the  protoplast  of  the 


244 


STRUCTURE   AND   LIFE   HISTORIES 


antheridium  continues  to  divide  until  as  many  as  1,500  or 
more  sperms  are  formed  in  each  one.  Since  the  antheridia 
occur  in  groups  oisori  (Figs.  178  and  179),  the  total  num- 


FiG.  178. — Diclyola  dichotoma.  Cross-section  of  a  male  thallus,  show- 
ing the  comparative  development  of  the  antheridial  sori,  and  the  tufts  of 
hairs  which  are  scattered  over  the  frond,  a,  young  sorus;  h,  older  sorus; 
c,  sorus  opened.  The  sperms  have  been  set  free.  There  remain  only  the 
cells  which  form  the  involucre,  d,  tuft  of  very  young  hairs;  e,  tuft  of 
older  hairs;  /,  the  same  fully  developed.     X  about  35.     (After  Thuret.) 


SoOOiOSoo    0 


SgS; 


Qino- 


Fig.  179. — Dictyota  dichotoma.  A,  vertical  section  through  portion 
of  sorus,  showing  antheridia;  B,  sperms.  (Redrawn  from  J.  Lloyd  Wil- 
liams.) 


ber  of  sperms  formed  on  one  plant,  or  even  in  one  sorus, 
is  enormous.  The  oogonia  may  occur  singly  or  in  groups 
(Fig.  180). 


LIFE   HISTORIES   OF   ALGiE 


245 


230.  Discharge  of  Gametes. — As  in  Ascophyllum,  both 
kinds  of  gametes  are  set  free  at  the  same  time,  before 
fertilization.  Recent  studies  have  disclosed  the  very 
interesting  fact  that  their  discharge  occurs  at  rhythmic 
intervals  of  about  two  weeks,  synchronizing  with  the 
periods  of  high  and  low  tide.  The  advantages  of  this,  if 
any,  are  not  apparent,  and  the  periodicity  persists  in  plants 
placed  in  jars  of  sea-water  in  the  laboratory,  and  even 
with  branches  newly  developed  in  the  laboratory,  and  thus 


Fig.  180. — Dictyota  dichotoma  (Huds.)  Lamx.    Longitudinal  section  of  an 
oogonial  sorus.     (After  Bornet  and  Thuret.) 

never  (as  branches)  subjected  to  the  variations  of  the  tide. 
So  close,  however,  is  che  harmony  between  tidal  periods 
and  discharge  of  gametes,  that  the  exact  day  of  their  dis- 
charge, at  any  given  station,  can  be  predicted  (with  an 
error,  at  most,  of  only  one  day)  by  consulting  the  tide- 
tables  for  the  given  station. 

231.  Fertilization. — From  the  freshly  liberated  eggs 
there  diffuses  through  the  water  some  unknown  substance 
which  attracts  the  sperms.  The  latter  respond  to  this 
stimulus  by  swimming  toward  the  egg.     One  of  them 


246 


STRUCTURE    AND    LIFE   HISTORIES 


enters  it,  and  makes  its  way  through  the  cytoplasm  to  the 
egg-nucleus  (Fig.  181),  with  which  it  fuses,  thus  completing 
the  act  of  fertilization.  The  other  sperms  come  to  rest 
outside  the  egg,  and  finally  disintegrate.  If  eggs  liberated 
at  different  periods  occur  in  the  water  together,  the  sperms 
will  swim  toward  those  liberated  last,  in  preference  to  the 
others.  As  soon  as  fertilization  has  been  accomplished, 
the  oosperm  begins  to  divide,  and  develops  into  a  new 
plant ,  which  finally  comes  to  resemble  externally  the  ones 
that  bore  the  antheridia  and  oogonia. 


4  r ,/ 


Fig.  181. — Dictyota  dicholoma.  At  the  left,  section  of  newly  liberated 
egg;  «,  egg;  cyto,  cytoplasm  of  egg;  sp,  three  of  the  numerous  sperms  ap- 
proaching the  egg  to  fertilize  it.  At  the  right,  portion  of  a  section  of  an 
egg  after  one  of  the  sperms  (shown  at  the  right  of  the  egg-nucleus,  en)  has 
entered;  sp,  sperms  that  have  not  entered.  The  fusion  of  the  sperm  with 
the  egg  nucleus  has  been  delayed,  and  the  sperm  has  greatly  increased  in 
size.     (Redrawn  from  J.  Lloyd  Williams.) 


232.  Asexual  Reproduction. — The  plants  that  develop 
from  fertilized  eggs  never  bear  antheridia  and  oogonia,  but 
non-motile,  asexual  spores  only.  These  are  set  free  at 
irregular  intervals,  and  never  at  rhythmic  periods  like  the 
gametes.  They  are  produced  by  two  successive  divisions 
of  a  spore-mother-cell,  and  thus  occur  in  groups  of  four 
(tetrads) ;  the  plants  that  bear  them  are  commonly  referred 


LIFE   HISTORIES    OF   ALG^E 


247 


to  as  tetrasporic  plants.  When  the  tetraspores  are  set  free 
they  soon  become  attached  to  some  solid  object,  and,  like 
the  fertilized  eggs,  develop  into  plants  that  externally 
resemble,  at  maturity,  those  bearing  tetraspores.  Thus, 
the  plants  produced  by  the  fertilized  eggs  and  by  the  tetra- 
spores closely  resemble  each  other  in  all  vegetative  characters; 
they  differ  externally  only  in  the  kind  of  reproductive 
organs  they  bear. 

233.  Alternation  of  Generations. — Although  the  Dic- 
tyota  plants  developed  from  zygotes  and  spores  look  alike, 


/. 


Fig.  182. — Dictyota  dichotoma.  A,  Vertical  section,  transverse  to  the 
axis  of  the  thallus,  showing  a  polar  view  of  the  nuclear  plate  in  the  first 
division  of  the  antheridium.  C,  Similar  view  of  the  first  division  of  the 
oogonium;  B,  similar  view  of  the  first  nuclear  division  of  the  fertilized  egg. 
Note  that  the  reduced  (haploid)  number  of  chromosomes  in  A  and  C  is  16, 
while  the  fertilized  egg  (B)  shows  the  diploid  number  (32).  (Redrawn 
from  J.  Lloyd  Williams.) 

it  is  obvious  that  the  products  of  the  tetraspore,  since  they 
bear  gametes,  and  never  spores,  are  gametophytes;  and 
the  products  of  the  fertilized  egg,  since  they  bear  spores 
only,  and  never  gametes,  are  sporophytes.  These  facts 
have  only  recently  been  established  by  careful  experi- 
mental cultures.  There  is  thus  a  true  alternation  of 
generations,  although,  in  marked  contrast  to  the  ferns  and 
mosses,    the   plant   bodies   of   the    two   generations   are 


248  STRUCTURE   AND   LIFE   HISTORIES 

vegetatively  alike.  Obviously  we  would  expect  all  the 
cells  of  the  gametophyte  to  bear  the  reduced  or  haploid 
number  of  chromosomes,  and  all  the  cells  of  the  zygote  or 
sporophyte,  the  diploid  number.  Such  is  the  case.  Fig. 
182, C,  shows  a  section  of  an  oogonium  with  the  nucleus 
in  its  first  division,  and  the  number  of  chromosomes  (16) 
may  be  easily  counted.  In  Fig.  182,  A,  is  shown  the  divid- 
ing nucleus  of  the  antheridium  also  with  16  chromosomes. 
In  the  fertilized  egg  (Fig.  182,  B)  the  diploid  number  (32) 
may  readily  be  counted.  Since  Dictyota  is  dioecious,  we 
must  infer  that  the  tetraspores,  though  looking  alike  ex- 
ternally, are  fundamentally  different  internally,  since  part 
of  them  give  rise  to  antheridial  or  male  plants^  and  part 
to  oogonial  or  female  plants. 

234.  Reduction. — Since  all  the  cells  of  the  sporophyte 
(tetrasporic  plant)  contain  the  diploid  number  of  chromo- 
somes, and  all  the  cells  of  the  gametophytes  the  reduced 
number,  the  question  naturally  arises,  where  does  re- 
duction take  place?  It  occurs  in  the  two  divisions  that 
result  in  the  formation  of  the  tetraspores;  each  of  the 
latter  possess  the  reduced  number,  whereas  the  spore- 
mother-cell  is  diploid.  From  this  we  learn  that  reduction 
may  occur  at  different  points  in  the  life-cycle  in  different 
forms.  In  Ascophyllum  it  occurs  in  the  nuclear  divisions 
immediately  preceding  the  formation  of  the  gametes;  in 
Dictyota,  immediately  preceding  the  formation  of  the 
spores,  while  between  reduction  and  the  formation  of 
gametes  occur  the  innumerable  cell-divisions  that  give 
rise  to  the  body  of  the  gametophyte. 

235.  Life-cycle  of  Dictyota. — The  life-cycle  of  Dictyota 
may  be  diagrammed  thus : 


LIFE   HISTORIES    OF   ALG^  249 

OUTLINE  OF  LIFE  HISTORY  OF  DICTYOTA 

Antheridial-plant  Oogonial-plant 

(Male  gametophyte)  (Female  gametophyte) 

4,  4. 

Antheridium  Oogonium 

4-  4. 

Sperm       Fertilization 


Oosperm  (zygote) 

4.4, 

Tetraspo  ric-plant 
(Sporophyte) 

II 

Sporangium 

Spore-mother-cell 

\,           \.             \,  \.       \Reduction 

Tetra-     Tetra-     Tetra-  Tetra- 

spore       spore       spore  spore 

4^  4*  4"  4' 

Male        Male      Female    Female 

gameto-  gameto-  gameto-  gameto 

phyte       phyte       phyte       phyte 

236.  Effect  of  Environment. — How  is  the  similarity  in 
external  appearance  of  the  alternating  generations  of 
Dictyota  to  be  explained?  We  cannot  say  for  certain. 
The  two  generations  start  from  reproductive  cells  (zygotes 
and  spores)  that  are  profoundly  unlike  in  their  internal 
organization,  but  it  must  be  borne  in  mind  that  both 
generations  develop  under  practically  identical  external 
conditions — the  surrounding  ocean-water,  with  alternate 
exposure  and  immersion  at  low  and  high  tides.  This 
would  tend  to  influence  the  plant  bodies  alike,  and  has 
been  suggested  by  some  botanists  as  an  explanation  of  the 
resemblance  of  the  two  kinds  of  generations.  In  the 
case  of  ferns  and  mosses  the  environment  of  the  two 


250  STRUCTURE    AND    LIFE    HISTORIES 

generations  is  not  alike.  The  sporophyte,  for  example, 
begins  life  as  a  parasite  on  the  gametophyte,  while  the 
gametophyte  leads  an  independent  existence  from  the 
start.  The  reader  may  also  recall  other  differences.  The 
question  we  have  here  raised,  however,  is  a  very  funda- 
mental one,  and  will  be  further  discussed  in  Chapter 
XXII. 

ULOTHRIX 

237.  Habitat. — The  genus  Ulothrix  occurs  everywhere, 
from  pole  to  pole,  in  fresh  water. 

238.  Description. — The  plant  body  (Fig.  262)  is  usually 
a  simple  thread  of  cells,  though  in  exceptional  cases  the 
cell-divisions  result  in  a  cell-plate  instead  of  a  thread.^  All 
the  cells  are  similar  in  appearance  and  structure,  except 
the  basal  one,  by  which  the  plant  is  attached  to  some  solid 
body.  This  cell  is  somewhat  larger  than  the  others, 
possesses  less  pigment,  and  is  suitably  modified  to  serve 
as  an  organ  of  attachment,  or  hold-fast.  The  protoplast 
of  each  cell  possesses  one  nucleus,  surrounded  by  the  green, 
cylindrical  chloroplast.  No  other  pigment  occurs,  and 
therefore  Ulothrix  belongs  to  the  Chlorophyceae,  or  green 
algae.  Photosynthesis,  of  course,  takes  place,  and  a 
portion  of  the  photosynthate  may  become  transformed 
into  starch,  the  presence  of  which  is  easily  demonstrated 
by  the  usual  test  with  iodine. 

239.  Asexual  Reproduction. — Every  cell  of  the  plant, 
except  the  hold-fast,  is  capable  of  functioning  as  a  re- 
productive cell,  and  two  methods  of  reproduction  are 
common.     In  one   case  the  entire  protoplast  of  a  cell 

^  These  two  forms  of  plant  bodies  are  sometimes  designated  by  the 
terms  "linear  aggregate"  (a  filament)  and  "superficial  aggregate"  (a 
plate). 


LIFE    HISTORIES    OF    ALG^  25 1 

becomes  organized  into  from  one  to  eight  motile  spores 
(zoospores,  or  swarm-spores),  each  with,  four  cilia  and  a 
red  ''eye-spot."  The  zoospore  escapes  by  swimming 
through  a  small  opening  in  the  cell-wall,  attaches  itself 
by  its  ciliate  end,  and  by  a  series  of  cell-divisions  produces  a 
new  filament  like  the  one  from  which  it  came.  In  this 
case  it  is  evident  that  the  zoospore,  reproducing  without 
cell-fusion,  is  an  asexual  spore,  and  that  the  mother-cell 
from  which  it  came  functioned  as  a  sporangium. 

240.  Sexual  Reproduction. — Other  cells  of  the  same 
filament  that  produced  the  asexual  zoospores,  may,  by 
successive  nuclear  and  cell-divisions,  become  divided  into 
as  many  as  1 6  to  64,  or  even  more,  independent  motile  cells, 
each  with  a  red  "eye-spot,"  but.  with  only  two  cilia. 
Like  the  zoospores,  they  escape  by  swimming  through  an 
opening  in  the  wall  of  the  mother-cell.  Occasionally  one 
of  them  comes  to  rest  and  begins  germination,  but  the 
process  never  continues  far  enough  to  produce  a  new  plant. 
More  commonly,  two  of  these  cells  come  together  and  fuse, 
showing  that  they  are,  in  reality,  gametes.  Since  the 
gametes  are  similar  in  size  they  are  termed  equal  gametes, 
or  isogametes.  The  fusion  of  two  isogametes  is  called 
conjugation,  to  distinguish  it  from  the  fusion  of  un- 
equal gametes;  it  is  essentially  the  same  as  fertilization. 
Whether  or  not  reduction  occurs,  in  the  nuclear  and  cell- 
divisions  that  result  in  the  formation  of  the  isogametes, 
is  not  known. 

241.  Germination. — After  the  fusion  of  the  gametes  the 
zygote  at  once  begins  to  increase  in  size,  but  soon  its  cell- 
wall  becomes  thickened,  and  then  the  protoplast  divides 
into  a  number  of  swarm-spores,  each  of  which,  when  set 
free,  may  develop  into  a  new  plant. 


252 


STRUCTURE   AND   LIFE   HISTORIES 

OUTLINE  OF  LIFE  HISTORY  OF  ULOTHRIX 

Plant  Body 
(Undifferentiated  into  gametophyte  and  sporophyte  stages) 


i  i 

i  i 

Physiological 

Physiological 

Sporangium 

Gametangium 

4-  4- 

4,                 >t       )  Reduction 

Zoospore 

Isogamete                Isogamete 

I  i 

^^-^„^Conj  ligation ^^0^ 

Plant  Body 

^"^■^-^   l^X^^ 

(Undifferentiated  into 

Zygote 

gametophyte  and  sporo- 

I   I 

phyte  stages) 

Swarm-spores 

4.  4. 

Plant  Body 

PLEUROCOCCUS 

242.  Habitat. — Everyone  is  familiar  with   the  green 
layer  found  on  the  outer  surface  of  the  bark  of  trees,  on 


Fig.  183. — Individual  plants  of  green  slime  {Pleurococcus  vulgaris),  show- 
ing the  tendency  of  the  cells  to  remain  attached  after  cell-divisions,  thus 
causing  transitions  from  a  one-celled  to  a  multi-cellular  plant. 

wooden  fences,  and  the  moist  shaded  surfaces  of  stone 
steps  or  rocks.  Careful  observation  will  show  that  this 
crool  is  largely  due  to  the  presence  of  countless  numbers 


LIFE  HISTORIES   OF   ALG^ 


253 


of  a  tiny  green  plant,  called  Pleurococcus,  usually  of  the 
species  Pleurococcus  vulgaris.  The  individual  plants  are 
so  small  that  they  may  be  seen  only  with  the  aid  of  the 
microscope  (Fig.  183).  An  examination  of  the  trees  in 
any  given  locality  will  disclose  the  fact  that  Pleurococcus 
prefers  one  side  of  the  tree  to  the  other,  and  that  the 
choice  of  sides  has  a  direct  relation  to  light,  temperature, 
or  moisture — one  or  all. 


Fig.  184. — Pleurococcus  vulgaris.  Sections  of  one-,  two-,  and  four-celled 
plants,  showing  the  nuclei  and  the  large  chlorophyll  bodies  (chb)  to  which 
the  green  color  of  the  plants  is  due.  In  D,  the  larger  chloroplast  is  shown 
in  perspective.  (Camera  lucida  drawings  from  a  microscopic  preparation 
by  E.  W.  Olive.) 


243.  Structure. — No  plant  structure  could  be  much 
simpler  than  that  of  Pleurococcus,  for  the  plant  body  is  a 
single  cell,  the  simplest  organic  unit  capable  of  independent 
existence.  The  protoplast  possesses  a  well-defined  nucleus 
and  a  chloroplast,  and  is  surrounded  by  a  cellulose  cell- 


2  54  STRUCTURE    AND    LIFE    HISTORIES 

wall    (Fig.    184).     We   have  here,  in  fact,   a  unicellular 
organism. 

244.  Reproduction. — The  reproduction  of  Pleurococcus 
consists  merely  of  the  processes  of  cell-division,  and  the 
final  separation  of  the  daughter-cells.  The  latter  may 
adhere  until  several  divisions  have  occured,  but  eventually 
the  middle  layer  of  the  cell-wall  common  to  the  two 
adjacent  daughter-cells  is  dissolved,  probably  by  an 
enzyme.  The  cells  then  separate  from  one  another,  and 
become  independent  plants,  increasing  in  size,  and  soon 
repeating  the  simple  reproductive  process  just  described. 

245.  Simplicity  of  Pleurococcus. — The  structure,  life- 
processes,  and  life-relationships  of  plants  could  hardly 
find  a  more  simple  expression  than  in  Pleurococcus. 
Morphologically  the  green  plant  is  here  reduced  to  its 
lowest  terms.  There  is  no  differentiation  into  parts — 
no  hold-fast,  roots,  or  rhizoids,  no  shoot,  no  special 
reproductive  organs.  From  the  standpoint  of  physiology, 
every  essential  function  is  performed  by  the  one  cell — 
absorption  of  water  and  dissolved  nutrient  substances 
by  osmosis  through  the  cell- wall  and  plasma-membrane; 
photosynthesis,  with  entrance  of  carbon  dioxide  and 
exit  of  oxygen  by  diffusion  through  the  membrane  and 
cell- wall;  respiration,  with  the  accompanying  exchange 
of  gases  in  the  same  way;  digestion,  assimilation,  and 
growth,  resulting  finally  in  reproduction  by  the  division 
of  the  entire  plant  body  into  new  individuals.  So  far 
as  known,  such  processes  as  cell-fusion  (in  fertilization 
or  conjugation),  reduction,  and  alternation  of  genera- 
tions are  entirely  absent.  Pleurococcus  is  a  generalized 
plant,  with  almost  no  division  of  physiological  labor. 
At  one  time  the  entire  plant  body  functions  vegetatively, 


LIFE   HISTORIES   OF   ALG^  255 

that  is,  for  the  maintenance  of  the  individual;  at  another 
time  the  entire  plant  body  functions  reproductively, 
that  is,  for  the  maintenance  of  the  race  to  which  the 
individual  belongs. 

246.  Life  History  of  Pleurococcus. — The  simple  life- 
history  of  Pleurococcus  may  be  briefly  indicated  as  follows : 

OUTLINE  OF  LIFE  HISTORY  OF  PLEUROCOCCUS 

Unicellular  plant 

4- 

Cell-divisions 

i 

Temporary  cell-aggregates 

I 

Separation      of      daughter-cells 

Ar  Ar  Ar  •\' 

Unicellu-  Unicellu-  Unicellu-  Unicellu- 
lar plant  lar  plant  lar  plant  lar  plant 


CHAPTER  XIX 
LIFE  HISTORIES  OF  FUNGI 

247.  The  Groups  of  Ftingi. — We  are  all  more  or  less 
familiar  with  fungi,  as  represented  by  the  common 
molds,  mildews,  toadstools,  and  mushrooms.  They 
are  all  plants  without  chlorophyll,  and  are  therefore 
dependent  upon  green  plants  for  their  nourishment. 
The  Greek  word  for  fungi  is  mycetes,  and  this  word 
terminates  the  names  of  the  various  groups,  as  follows: 

1.  Phycomycetes,  alga-Hke  fungi;  so-called 
because  they  closely  resemble  certain 
algae,  except  for  the  lack  of  chlorophyll. 

2.  Ascomycetes,  sac-fungi;  so-called  because 
their  asexual  spores  are  formed  in  tiny 
sacs  (asci) 

3.  Basidiomycetes,  with  spores  borne  on 
little  club-shaped  hy^Yidd,  or  basidia.  In- 
clude the  smuts,  rusts,  and  mushrooms. 

AN  ALGA-LIKE  FUNGUS  (RHIZOPUS) 

248.  Habitat. — Everyone  is  acquainted  with  "bread 
mold,"  a  plant  without  chlorophyll,  and  having  a  fila- 
mentous plant  body.  There  are  many  kinds  of  fila- 
mentous fungi,  more  or  less  closely  related  to  Rhizopus, 
and  growing  on  various  substances  or  "substrata." 
They  all  agree  in  at  least  three  points:  (i)  they  are  always 
filamentous;  (2)  they  never  possess  chlorophyll;  (3)  they 
always  grow   on  some  organic  substratum.    The  sub- 

256 


LIFE    HISTORIES    OF    FUNGI  257 

stratum  is  also  usually  moist.  Partly  as  a  result  of 
their  presence,  the  substratum  on  which  they  grow  is 
usually  disintegrating  with  decay.  From  these  facts 
Rhizopus  is  called  a  saprophyte}  The  most  common 
species  is  Rhizopus  nigricans. 

249.  How  Obtained. — Rhizopus,  or  almost  any  other 
filamentous  fungus,  may  be  readily  obtained  by  sowing 
its  spores  on  a  suitable  substratum.  But,  unlike  the 
higher  plants,  Rhizopus  may  ordinarily  be  obtained  merely 
by  exposing  moist  bread,  or  other  nutritive  substance,  to 
the  air.  In  the  course  of  time,  without  one's  sowing  any 
spores,  colonies  of  the  plant  will  appear,  and  this  clearly 
demonstrates  the  very  interesting  fact  that  these  spores 
are  always  floating  in  the  air  in  greater  or  less  abundance. 
When  the  moist  bread  is  exposed,  some  of  the  floating 
spores  come  to  rest  upon  it,  and  there,  under  favoring 
conditions  of  moisture,  temperature,  and  light  they 
germinate,  and  develop  new  plants. 

250.  Description  of  Plant  Body. — The  plant  body  of 
Rhizopus  (Fig.  185)  consists  of  a  slender,  thread-like 
filament,  called  the  mycelium,  branching  freely,  devoid 
of  chlorophyll,  and  growing  over  the  surface  of  the 
substratum.  The  threads  of  mycehum  are  termed 
hyphce.  Careful  examination  with  the  microscope  dis- 
closes the  fact  that  the  hyphae  are  largely  or  wholly 
aseptate,  that  is,  not  divided  by  cross-walls  or  septa. 
The  plant  body,  therefore,  consists  of  one  cell,  though 
there  are  several  to  many  nuclei  distributed  through  the 
cytoplasm;    such    a    structure    is    termed    a   coenocyte} 

^  From  the  Greek,  sapros  (aairpos),  rotten,  +  phyton  {ipvrSv),  a  plant. 
2  Greek,  koivos  {koinos),  common,  -1-  kvtos  (kutos),  a  hollow  (cell). 
17 


258 


STRUCTURE   AND    LIFE   feCSTORIES 


The  protoplasm  appears  to  be  streaming  in  a  constant 
current  in  one  direction,  and  this  is  thought  to  be  due  to 
the  evaporation  of  water  from  surfaces  more  exposed  to 
the  air,  and  the  intake  of  more  water  from  the  substratum, 
by  osmosis. 


Fig.  185. — Bread  mold  {Rhizopus  nigricans).  A,  older  plant;  myc, 
mycelia;  sph,  sporangiophore;  sp,  sporangium;  st,  stolon  produced  by  A, 
and  giving  rise  at  its  tip  to  a  new  plant,  B.     Greatly  enlarged. 


251.  Secretion  of  a  Powerful  Poison. — In  the  course 
of  some  experiments,  made  in  order  to  determine  the 
cause  of  sex  in  the  mucors,  Blakeslee  and  Gortner  in- 
jected into  the  ear  of  a  healthy  rabbit  some  of  the  juice, 
squeezed  out  of  the  mycelium  of  Rhizopus  nigricans. 
To  their  great  surprise  the  animal  died  almost  instantly, 
before  the  injection  was  completed.  Further  experi- 
ments clearly  demonstrated  that  Rhizopus  contains  a 
powerful  poison  (or  toxin)  ^  which  is  soluble  in  water, 
but  which  produces  its  effect  only  when  introduced  into 
the  circulatory  system.  When  this  expressed  juice  was 
fed  to  the  rabbits,  or  when  they  ate  the  mycelium,  no 


LIFE    HISTORIES    OF   FUNGI  259 

harmful  result  followed.  The  discoverers  of  the  toxin 
suggest  that  the  discovery  may  possibly  throw  light  on 
''pellagra"  and  some  of  the  destructive  diseases  of  cattle, 
such  as  "cornstalk-disease,"  and  the  "horse-disease," 
prevalent  in  some  of  our  western  states,  and  for  some 
time  thought  to  be  due  to  some  food-impurity. 

262.  Vegetative  Mtiltiplication. — At  various  points  the 
mycelium  produces  one  or  more  upright,  aerial  hyphae, 
15  to  20  millimeters  high,  and  of  larger  diameter  than  the 
mycelial  hyphae.  One  or  more  of  these  hyphse  eventu- 
ally bend  over  and  grow  for  a  short  time  along  the 
surface  of  the  substratum,  like  the  stolons  or  "runners" 
of  such  plants  as  strawberries.  Finally,  at  their  free  ends 
there  develop  slender  hyphse  that  penetrate  the  sub- 
stratum, like  rhizoids,  and  another  group  of  the  larger, 
aerial  hyphae,  one  or  more  of  which  may  repeat  the  process 
just  described.  The  formation  of  these  stolons  suggested 
a  former  name  of  the  plant,  viz.,  Mucor  stolonifer. 

253.  Asexual  Reproduction. — At  the  tip  of  each  up- 
right hypha  there  develops  a  globular  sporangium,  and 
hence  the  hypha  is  called  a  sporangiophore  (Fig.  i86).  The 
protoplasm  is  constantly  streaming  up  the  sporangiophore, 
getting  more  and  more  dense  toward  the  sporangium,  and 
contains  large  numbers  of  nuclei.  Further  down,  to- 
ward the  base  of  the  sporangiophore,  the  protoplasm 
becomes  thinner  and  thinner,  and  the  nuclei,  fewer  in 
number,  finally  disintegrate.  In  the  meantime  the  tip 
of  the  sporangiophore  increases  in  size  until  the  globular 
sporangium  is  formed,  rich  in  cytoplasm  and  nuclei. 
By  degrees  the  protoplasm  becomes  more  dense  toward 
the  periphery,  and  gradually  thinner  toward  the  central 
portion,  where  large  numbers  of  vacuoles  develop.     Soon 


26o 


STRUCTURE    AND    LIFE    HISTORIES 


a  well-marked  cleft  separates  the  denser  peripheral  pro- 
toplasm from  the  thinner  central  portion,  which  now 
develops  into  a  little  column,  ox  columella  (Fig.  i86).    In  the 


Fig.  1 86. — Rhizopus  nigricans,  i,  Young  sporangium,  showing  cyto- 
plasm and  nuclei  streaming  up  the  sporangiophore  into  the  sporangium 
and  out  toward  the  periphery.  2,  Sporangium  of  nearly  full  size.  The 
differentiation  between  the  dense  peripheral  and  the  looser  central  plasms 
is  clearly  shown.  Nuclei  are  still  passing  from  the  central  to  the  periph- 
eral regions  along  the  fine  strands  of  cytoplasm.  X  about  200.  (After 
D.  B.  Swingle.)     (Cf.  Figs.  187  and  188.) 

meantime  innumerable  furrows  develop  in  the  denser  plasm, 
first  at  the  periphery  and  then  working  their  way  in  from 
the  columella  cleft.     By  the  time  the  two  sets  of  furrows 


LIFE    HISTORIES    OF    FUNGI 


261 


meet,  the  dense  protoplasm  has  been  cut  up  into  innumer- 
able, more  or  less  angular  masses,  each  surrounded  by  a 
plasma-membrane,  and  containing  from  two  to  six  nuclei 


Fig.  187. — RhizopHS  nigricans,  i,  Full-sized  sporangium,  showing 
layer  of  vacuoles  nearly  formed  along  the  inner  surface  of  the  denser  plasm, 
X  about  200.  2,  Section  passing  to  one  side  of  the  sporangiophore, 
showing  columella-cleft  being  formed  by  fusion  of  the  vacuoles  shown  in  i, 
and. by  a  furrow  from  the  base  of  the  sporangium,  X  about  200;  3,  Detail 
of  another  part  of  same  sporangium  as  shown  in  2,  showing  early  cleavage 
furrows,  X  about  850.     (After  D.  B.  Swingle.)     (Cf.  Figs.  186  and  188). 

each  (Fig.  187).  These  finally  become  entirely  separated 
from  each  other,  oval  in  shape,  and  surrounded  by  a  wall 
of  fungus  cellulose  (chitin?)  having  clearly  marked  ridges 
(Fig.  188).     They  are  asexual  spores,  but  differ  from  the 


262 


STRUCTURE    AND    LIFE   HISTORIES 


spores  of  ferns,  mosses,  liverworts,  and  algae  by  usually,  if 
not  always,  possessing  more  than  one  nucleus.  They 
secrete  a  slimy  substance  in  which  they  are  imbedded. 


Fig.  188. — Rhizopus  nigricans,  i,  Section  of  sporangium,  showing 
cleavage  of  peripheral  cytoplasm  much  further  advanced  than  in  Fig.  187. 
Furrows  are  here  cutting  outward  from  the  columella  cleft,  X  about  200; 
2,  section  of  sporangium  in  which  the  spores  are  completely  formed, 
rounded  up,  and  surrounded  by  thin  walls.  The  columella  wall  is  also 
formed,  X  about  200;  3,  ripe  spores  in  their  living  condition,  showing 
variations  in  size,  and  ridges  on  their  walls,  X  over  350.  (After  D.  B. 
Swingle.)     (Cf.  Figs.  186  and  187.) 

When  ripe  the  wall  of  the  sporangium  bursts  open  (Fig. 
i8q),  and  the  spores,  thus  set  free,  float  away  through  the 
air  in  countless  millions.     So  widely  and  so  thickly  are 


LIFE    HISTORIES    OF    FUNGI 


263 


they  distributed  that  a  piece  of  moist  bread,  or  a  jar  of 
canned  fruit,  left  exposed  for  only  a  brief  period,  in  almost 
any  locality,  will,  as  noted  above,  soon  become  "moldy" 
from  the  growth  of  mycelium  produced  by  their  germina- 
tion. On  account  of  the  practically  universal  distribution 
of   ''bread  mold"   its   spores  are,   of  course,   commonly 


Fig.  iBg—Rhizoptis  nigricans.  A,  Young  sporangium,  showing  col- 
umella within;  B,  older  sporangium,  with  the  wall  removed,  showing  ripe 
spores  covering  the  columella;  C,  D,  views  of  the  collapsed  columella  after 
dissemination  of  the  spores. 

present  in  the  air  of  laboratories,  where  the  mold  is  a  great 
pest,  and  has  therefore  won  the  appropriate  title  of 
''laboratory  weed." 

254.  Sexual  Reproduction. — When  the  hyphae  of 
mycelia  derived  from  spores  of  different  sex-value 
are  intermingled  they  frequently  develop  short  lateral 
branches,  which  grow  out  toward  each  other  until  their 
tips  come  in  contact  (Fig.  190).  The  rich  protoplasmic 
contents  at  the  tips  are  cut  off  from  the  remainder  of 
the  coenocytic  mycelium,  the  walls  in  contact  with  each 
other  become  dissolved,^  and  the  two  protoplasmic  masses 
fuse.  This  will  be  recognized  as  conjugation;  the  fusing 
masses  of  protoplasm  are  isogametes,  and  the  cut-off  tips 
of    the    conjugating   branches    function    as   gametangia. 

^  Probably  by  enzyme  action,  though  this  has  not  been  actually 
demonstrated. 


264 


STRUCTURE    AND    LITE    HISTORIES 


After  the  zygote  is  formed,  the  outer  wall  thickens  and 
assumes  certain  external  characteristics,  easily  recog- 
nized. It  also  becomes  black  as  it  ripens,  and  this  fact 
has  given  rise  to  the  common  name,  "black  mold."  In 
this  condition  the  zygote  rests,  as  a  zygospore. 


Fig.  190. — Sexual  reaction  between  a  hermaphroditic  Mucor  and  (+) 
and  (  — )  races  of  a  dioecious  species.  Diagrammatic  representation  of  a 
Petri  dish  culture  showing  a  heterogamic  hermaphroditic  mucor  (^)  in 
the  center  separated  by  channels  on  either  side  from  the  (+)  and  (— ) 
races,  respectively,  of  a  dioecious  species.  Sp.,  sporangia  containing 
spores  by  means  of  which  the  plant  may  be  reproduced  nonsexually.  1-6, 
stages  in  development  of  a  hermaphroditic  zygospore  from  unequal  male 
and  female  gametes.  A,  sexual  reaction  between  a  (  — )  filament  and  fe- 
male gamete.  B,  sexual  reaction  between  a  ( +)  filament  and  male  gamete. 
C,  a  male  zygospore  formed  at  stimulus  of  contact  with  a  (+)  filament. 
(After  Blakeslee.) 

256.  Germination. — At  the  close  of  the  resting  period, 
and  under  favorable  conditions  of  temperature  and  mois- 
ture, the  zygote  germinates,  sending  out  an  erect  hypha, 
which  at  once  develops  a  globular  sporangium  at  its  apex. 
The  asexual  spores  from  this  sporangium  become  the 
starting  point  of  another  series  of  changes  like  those 
just  described. 

256.  Sexuality  of  Rhizopus. — It  is  well  known  that, 
while  conjugation  often  occurs  freely  between  mycelia 
from  different  spores,  in  other  cases  it  fails  entirely.  The 
explanation  of  this  was  not  known  until  about  eight  years 


LIFE    HISTORIES    OF    FUNGI  265 

ago,  when  it  was  discovered  that  there  are,  in  reality,  two 
unlike  strains  of  Rhlzoptis-mycelmm,  and  therefore  two 
kinds  of  spores.     The  two  kinds  of  mycelia  are  designated 


Fig.  191. — Petri-dish  culture  showing  dark  line  of  zygospores  between 
the  (+)  and  (  — )  strains  of  a  dioecious  species  of  mo\d,'C/i(>anephora  (Mc) 
and  white  lines  of  "imperfect  hybridization"  between  the  strains  of  this 
species  and  the  opposite  strains  of  JMucor  V  (Mv).  Note  the  total  absence 
of  zygospore  formation  between  two  (+)  or  two  (  — )  strains.  (After 
Blakeslee.) 

as  (+)  and  (— ).  When  the  interminghng  hyphae  are  of 
h'ke  strains,  either  all  (+)  or  all  (  — ),  conjugation  fails 
completely,  but  when  they  are  from  unlike  strains,  zygo- 
spores form  in  great  abundance.     This  fact  is  strikingly 


266 


STRUCTURE    AND    LIFE    HISTORIES 


shown  in  Fig.  191,  which  illustrates  the  result  of  growing 
mycelia  from  unlike  strains  side  by  side.  The  zone  where 
they  come  into  contact  is  sharply  defined  by  the  line  of 
black  zygospores,  resulting  from  conjugation. 

257.  Distinction  of  Sexes. — When  first  discovered,  the 
two  unlike  strains  were  designated  as  (+)  and  (— ), 
because  it  was  not  possible  to  decide,  with  certainty, 
which    was    male    and    which    female.     Externally   both 


Fig.  192. — Diagram  of  life-cycle  of  a  dioecious  mold. 

strains  looked  very  much  alike,  except  that  one  appeared 
to  be  vegetatively  more  vigorous  than  the  other.  Recent 
experiments  seem  to  indicate  that  the  vegetatively  more 
vigorous  (+)  strain  is  female,  while  the  less  vigorous 
(— )  strain  is  male.  Not  all  molds  are  dmcious,  like 
Rhizoi>tis,  In  some  species  the  mycelium  from  a  single 
spore  produces  conjugating  branches  of  both   (+)   and 


LIFE    HISTORIES    OF    FUNGI  267 

(— )    value,    which   fuse   and   form   a   zygospore.     Such 
mycelia  are,  of  course,  monoecious. 

The  life-cycle  of  a  dioecious  mold  is  illustrated  in 
Fig.  192. 

258.  Phycomycetes. — Rhizopus  nigricans  represents  the 
group  of  Phycomycetes,  lower  fungi,  often  called  "tube- 
fungi."  The  group  is  characterized  by  the  tubular  hyphae 
without  septa.  In  this  coenocytic  character  they  re- 
semble the  filamentous  or  tubular  green  algae  (such  as 
Vaucheria,  or  "green  felt"),  of  which  they  are  supposed 
to  be  degenerate  descendants. 

259.  Life -cycle  of  Rhizopus. — The  life-cycle  of  Rhizopus 
may  be  diagrammed  as  follows: 

OUTLINE  OF  LIFE  HISTORY  OF  RHIZOPUS 

(+)  Spore  (-)  Spore 

4-  I 

Mycelium  Mycelium 

i  i 

Gametangium  Gametangium 

i  4. 

(+)  Gamete  (— )  Gamete 

Conjugati 


Zygospore 

4.4. 

Germ-tube 

4,  I 

Sporangium 

4^  I  Reduction 


(-|-)  Spores  (  — )  Spores 

i  i 

(-I-)  Mycelium  (  — )  Mycelium 

i  4, 

Sporangium  Sporangium 

4,  4- 

(+)  Spore  (  — )  Spore 


268 


STRUCTURE    AND    LIFE    HISTORIES 


A  SAC-FUNGUS  (MICROSPHiERA) 

260.  Habitat  and  Structure. — The  fungus,  Micro- 
sphcera,  may  be  found  growing  on  the  leaves  of  the  lilac, 
and  is  commonly  called  ''powdery  mildew."     The  my- 


FiG.  193. — Lilac  mildew  {Microsphara  Alni).  The  white  areas  are  the 
mycelia  of  the  fungus,  growing  over  the  surface  of  the  leaf.  The  tiny 
black  dots  are  the  perithecia,  which  contain  the  asci. 


celium  is  septate,  and  forms  a  web  over  the  surface  of  the 
leaf  (Fig.  193),  sending  down  haustoria  into  the  epidermal 
cells,  to  secure  nourishment,  and  sending  up  short  branches 
(conidiophores),  each  bearing  a  chain  of  colorless  spores 

{conidia) . 


LIFE    HISTORIES    OF    FUNGI 


269 


261.  Reproduction  by  Asci. — In  late  summer  or  early 
fall  one  may  notice,  on  an  infected  leaf,  among  the  my- 
celium, tiny  black  dots  or  spheres  (Fig.  194),  whence  the 
name,  Microsphcera.  Examined  with  the  microscope,  these 
bodies  are  seen  to  bear  numerous  appendages,  branched 
at  the  end,  and  with  the  tips  of  the  branches  curved  back  to 
form  miniature  hooks  (Fig.  195).  When  these  little  spheres 
are  crushed,  or  when  they  burst  open,  they  are  found  to  con- 


FiG.    194. — Powdery  mildew  {MicrospJmra  Alni)  on  lilac  leaf.     An  in- 
fected area  from  the  leaf  in  Fig.  193,  greatly  magnified. 


tain  a  number  of  tiny  sacs  or  asci  (singular  ascus),  whence 
the  name  "sac-fungus,"  or  Ascomycete.  In  each  ascus  are 
a  number  of  spores  or  ascospores,  formed  from  the  con- 
tents of  the  ascus.  The  young  ascus  is,  therefore,  a  spore- 
mother-cell.  There  are  usually  eight  ascospores  in  an 
ascus,  but  the  number  may  vary. 

The  spherical  case  containing  the  asci  is  the  '^spore- 
fruit"  (ascocarp  or  perithecium) ,  and  results  from  the 
fusion  of  the  contents  of  an  antheridium  and  an  oogonium 


270 


STRUCTURE    AND    LIFE    HISTORIES 


(Fig.  185).  The  cells  from  this  point  on,  to  and  including 
the  formation  of  the  ascus,  are  diploid,  and  therefore 
constitute  the  sporophytic  generation. 


Fig.  195. — Lilac  mildew  {Microsphcera  Alni).  A,  perithecium,  with 
appendages;  B,  perithecium,  showing  asci  (a);  C,  an  ascus,  containing 
ascospores;  D,  conidiophore  (cph),  bearing  a  chain  of  conidia  (conidio- 
spores,  c.s);  E,  beginning  of  fertilization;  antli,  antheridium;  car,  carpo- 
gonium;  F,  later  stage  in  fertilization;  the  contents  of  the  antheridium  and 
carpogonium  have  fused;  /,  fusion  of  the  two  nuclei;  G,  germination  of 
ascospore  (a.s);  g.t,  germ  tube.     (£  and  F  after  R.  A.  Harper.) 


262.  Germination. — Reduction  occurs  during  the  forma- 
tion of  the  ascospores.  When  an  ascospore  germinates  it 
develops  directly  into  a  mycelium. 

263.  Life-cycle. — The  Hfe-cycle  of  Microsphcera  may 
be  tabulated  as  follows: 


UTE  HISTORIES  OF  FUNGI  27 1 

OUTLINE  OF  LIFE  HISTORY  OF  MICROSPH^RA 


Spore  (conidium) 

I 

Mycelium 

4. 

Antheridium^ 


Spore  (conidium) 

4. 

Mycelium 

i 

Oogonium* 


Zygote  in  ascocarp  (Sporophyte) 
Ascogenous  hyphae 

4.  4. 

Young  asci 

^        }  Reduction 
Ascospores   (Gametoph^'te') 

I 

Mycelium 

I  " 

Conidiophore 

4. 

Conidium   (Conidiospore) 


Fig.  196. — A  "cup-fungus,"  Peziza  sylvestris.     (Photo  by  F.  J.  Seaver). 

^  In  MicrosphcBra  there  is  only  a  slight  morphological  diflferentiation  of 
the  gametes. 


272 


STRUCTURE    AND    LIFE    HISTORIES 


264.  Other  Sac -fungi. — Between  25,000  and  30,000 
species  of  sac-fungi  have  been  described.  Some  of  them 
are  filamentous,  like  the  lilac-mildew,  while  some  are 
fleshy,  like  the  common  ''cup-fungi''  (Fig.  196).  The 
edible  morel,  Morchella  esculenta,  (Fig.  197)  is  an  Asco- 
mycete,  and  the  common  yeast  referred  to  in  the  chapter 
on  fermentation  (Fig.  60)  is  also  classed  here  because,  in 
one  of  its  methods  of  reproduction  the  unicellular  plant 


Fig.  197. — The  morel,  Morchella  esculenta.     (Photo  by  W.  A.  Murrill.) 

body  functions  as  a  spore-mother-cell,  the  protoplast 
becoming  organized  into  spores  (ascospores) ,  and  the  wall 
of  the  mother-cell  serving  as  an  ascus. 


A  "RUST"  FUNGUS  (WHEAT  RUST) 

265.  Importance. — One  of  the  most  important,  as  well 
as  most  difficult,  fungi  to  understand  is  the  wheat  rust 
(Puccinia  graminis).     This  fungus  is  important  because 


LIFE   HISTORIES    OF    FUNGI  273 

it  attacks  some  of  the  most  valuable  of  all  agricultural 
crops  (wheat,  oats,  rye,  barley,  etc.),  causing,  at  times, 
millions  of  dollars  worth  of  damage.*  It  is  difficult  to 
understand  because  it  is  heterxcious — that  is,  lives  alter- 
nately on  two  different  plants,  the  barberry  and  the  grain. 
For  many  years  the  form  on  the  grain  was  supposed  to 
be  quite  another  plant  from  that  on  the  barberry,  which 
was  called  jEcidium  herheridis. 

266.  Life  History. — a.  Red  Rust  Stage. — The  mycelium 
of  red  rust  (uredo  stage)  grows  between  the  cells  of  the 
stem  and  leaves  of  the  wheat,  or  other  grain,  and  finally 
during  the  summer,  numerous  sporophores,  bearing  red 
spores  {uredinios pores),  break  through  the  epidermis 
(Fig.  198),  producing  reddish  or  rusty-looking  dots  and 
lines,  whence  the  name  ''rust"  for  the  plant.  The  one- 
celled  uredinio-spores  are  easily  blown  by  the  wind  in 
great  numbers  to  other  wheat  plants,  where  they  germ- 
inate, and  thus  spread  the  rust  widely  and  rapidly. 

b.  Black  Rust  Stage. — In  late  summer  the  same  myce- 
lium develops  an  entirely  different  kind  of  spore,  two- 
celled,  and  black.  These  are  the  final  spores  of  the  season, 
the  teliospores  (or  teleutos pores),  and  in  them  the  nuclear 
fusions  occur.  They  rest  over  winter,  and  germinate 
the  following  spring,  each  cell  usually  sending  forth  a 
hypha  commonly  composed  of  four  cells,  which  constitute 
the  hasidium  (promycelium) .  Each  of  these  cells  produces 
a  tiny  sporophor.e,  bearing  at  its  tip  a  single  basidiospore 
(sporidium).     Reduction  occurs  during  germination. 

c.  Barberry  Stage. — The  basidiospores  are  blown  by  the 

*The  financial  loss  from  wheat-rust  in  the  United  States  amounted 
to  $67,000,000  in  1891;  in  1904  the  loss  in  North  Dakota,  South  Dakota, 
and  Minnesota  alone  was  estimated  at  $25,000,000. 
18 


274 


STRUCTURE    AND    LIFE   HISTORIES 


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LIPE   HISTORIES    OF   FUNGI  275 

wind  to  the  leaves  of  nearby  barberry  bushes,  where  they 
germinate.  The  resulting  myceHum  penetrates  the  epi- 
dermis, and  ramifies  between  the  cells  of  the  leaf  paren- 
chyma, from  which  it  absorbs  nourishment.  Under 
certain  circumstances,  as  for  example  in  Austraha  where 
barberries  do  not  grow,  the  barberry  stage  is  eliminated. 
Soon  this  mycehum  develops  a  reproductive  structure 
called  the  pycnium  (pycnidium  or  spermagonium),  which 
breaks  through  the  upper  epidermis  and  produces  innu- 
merable pycnios pores  (spermatia).  The  real  nature  of 
this  structure  is  not  known. 

OUTLINE  OF  LIFE  HISTORY  OF  WHEAT  RUST 
Rust  on  Grain 

,~       TT  TT 

Urediniospores  (early  summer)        Teliospores  (late  summer) 

4-4-  .  J-i 

Germination  Rest  (Nuclear  fusion) 

Mycelium  Germination 

ii  4-4- 

Urediniospores  Basidium 

4-4'  4'        \Redtiction 

Germination  Sterigmata 

ii  4. 

Etc.  Basidiospores  (sporidia) 

I 

To  barberry 

4.         4. 

Pycnia     ^cia  [Cell-fusion) 

I         i4. 

?  ^ciospores 

Rust- on  grain 

Near   the   lower   epidermis   are   formed   the   ''cluster- 
cups,"  or  (Ecia^  (aecidia).     They   break  through   to   the 
^  Sing,  oscium. 


276 


STRUCTURE    AND    LIFE    HISTORIES 


outer  surface,  and  the  countless  numbers  of  ceciospores 
are  carried  by  the  wind  to  nearby  wheat,  where  they,  in 
turn,  germinate,  and  the  complicated  series  of  events 
begins  again. 

267.  Diagram   of  Life   History. — The   Kfe   history   of 
Puccinia  graminis  may  be  outlined  as  shown  on  page  275. 

A  FLESHY  FUNGUS  (AGARICUS) 

268.  Habitat. — Fleshy    fungi    are    found    widely    dis- 
tributed,   growing   in    the    soil    of    fields,   pastures,    and 


Fig.  199 — Meadow  mushroom  {Agaricus  campestris  L.).  A,  view 
showing  under  side  of  pileus;  g,  gills;  a,  annulus,  or  remains  of  the  veil 
attached  to  the  stipe;  B,  side  view;  s,  stipe;  a,  annulus;  p,  margin  of  pileus, 
showing  at  intervals  the  remains  of  the  veil.     (After  W.  A.  Murrill.) 

woods,  or  on  tree  trunks,  decaying  logs,  and  elsewhere. 
The  familiar  edible  mushroom  (Agaricus  campestris), 
as  its  name  implies,  grows  commonly  in  meadows,  and 
is  hence  often  called  the  "meadow  mushroom'^  (Fig.  199)- 
One  of  the  most  poisonous  species,  Amanita  phalloides, 


LIFE    HISTORIES    OF   FUNGI  277 

(Fig.  200),  resembles  Agaricus  superficially  and  is  often 
mistaken  for  it.     Amanita  always  has,  at  the  base  of  the 
stalk,  a  cup,  which  Agaricus  lacks. 
269.  Description. — The  body  of  Agaricus  consists  of  a 


Fig.  200. — The  deadly  amanita,  Amanita  phalloidcs.     Note  the  cup  at 
the  base  of  the  stipe.      (Photo  by  E.  M.  Kittredge.) 

short  fleshy  stalk  (the  stipe),  having  numerous  root-like 
hyphge  {rJiizomorphs)  penetrating  the  soil  from  its  lower 
end,  and  bearing  at  its  upper  end  an  umbrella-shaped 
expansion,  the  pileus.  On  the  under  side  of  the  pileus  are 
numerous  thin  lamellae  or  gills.     The  stalk  and  pileus  are 


278 


STRUCTURE   AND    LIFE    HISTORIES 


composed  of  innumerable  hyphae  (Fig.  201),  closely  asso- 
ciated together  in  a  pseudo-tissue,  but  there  is  no  real 


Fig.  201. — Coriinarhis  cinnamomeus.  Above,  longitudinal  section 
through  stipe  and  pileus,  showing  gills  (g);  below,  longitudinal  section 
through  primary  and  secondary  gills;  h,  hymenium  or  sporogenous  surf  ace. 
(After  Gertrude  E.  Douglas.) 

tissue-differentiation,  such  as  occurs  in  the  mosses  and 
ferns  and  higher  plants,  though  the  external  layer  may 
usually  be  peeled  off  with  ease. 
270.  Reproduction.— If  the  pileus  of  Agaricus  is  placed 


LIFE    HISTORIES    OF    FUNGI 


279 


Fig.  202. — Spore-print  of  the  fly  agaric  {Amanita  muscaria  L.).  The 
spore-print  is  made  by  laying  the  pileus,  gills  downward,  on  a  sheet  of 
black  paper.  The  white  spores  fall  onto  the  paper,  making  the  print. 
(Photo  by  W.  A.  Murrill.) 


Fig.  203. — Fruiting  surface  (hymenium)  of  a  mushroom  {Agaricus). 
m,  hyphse  of  the  trama  and  sub-hymenium;  6,  basidium;  st,  sterigma;  sp, 
spores.     (Diagrammatic.) 


28o  STRUCTURE    AND    LIFE    HISTORIES 

with  the  gills  down  over  a  piece  of  white  paper,  and  left 
for  a  few  hours,  a  dark  purplish  deposit  will  form  in  lines 
under  each  gill.  With  Amanita  the  color  is  white.  This 
deposit  (the  ''spore-print ")  is  composed  of  spores,  shed  from 
the  surfaces  of  the  gills  (Fig.  202).     Microscopic  examina- 


FiG.  204. — •The  meadow  mushroom  {Agaricus  campestris,  var.  Colum- 
bia.) Young  fruiting  bodies  (carpophores).  The  mycelial  hyphae  are  in 
the  substratum.     (Photo  by  G.  F.  Atkinson.) 

tion  discloses  the  fact  that  the  gills  are  composed  of  a  net- 
work of  hyphae.  Their  surface  is  covered  with  innumer- 
able short,  thick,  club-shaped  bodies,  filled  with  proto- 
plasm (Fig.  203 ) .  These  are  the  basidia  (singular  basidium) . 
Fungi  which  bear  basidia  are  grouped  together  as  Basidio- 
mycetes.     At  the  tip  of  each  basidium  are  two  tiny  projec- 


LIFE    HISTORIES    OF   FUNGI 


281 


tions,  the  sterigmata  (singular,  sterigmaY,  and  on  the  end 
of  each  sterigma  is  a  spore  (basidiospore).  In  the  early 
stages  of  development  the  gills  appear  white,  at  maturity 
they  are  purplish,  and,  at  about  the  time  the  spores  are 
shed,  they  become  dark  brown.  Although  many  pains- 
taking experiments  have  been  made  in  order  to  secure  the 
germination  of  the  spores,  no  one  has  ever  been  successful. 
Whether  or  not  the  mushroom  is  produced  from  spores 
in  nature  we  can  only  conjecture.  No  sexual  organs  have 
ever  been  discovered  on  any  of  the  fleshy  Basidiomycetes. 


Fig.  205. — The  common  mushroom  [Agaricus  campestris).     Young  "but- 
tons."    (Photo  by  G.  F.  Atkinson.) 

271.  Vegetative  Propagation.— The  meadow-mushroom 
is  propagated  for  food  from  "bricks"  of  mycelium,  or 
''spawn"  that  may  be  purchased  from  seedsmen.  When 
portions  of  these  ''bricks"  are  placed  in  suitably  prepared 
soil,  under  favorable  conditions  of  moisture  and  heat, 
the  mycelium  resumes  its  growth,  ramifying  in  all  di- 
rections through  the  soil.  At  numerous  points  on  the 
mycehum  tiny  little  "buttons"  form  (Fig.  204).     As  the 

^  In  most  of  the  gill-bearing  fungi  there  are  four  sterigmata  on  each 
basidium,  as  shown  in  Fig.  203. 


282 


STRUCTURE    AND    LIFE   HISTORIES 


Fig.  206. — A  pore-bearing  fungus  {Poly poms  hetulimis).     Under  (fruit- 
ing) surface,  showing  the  pores. 


^  Fig.  207.— Giant  puflfballs  {Lycoperdon  giganteum  Botsch.).  Note  the 
size  of  the  four  specimens  as  compared  with  the  egg  and  fruit  in  the  center. 
(After  W.  A.  Murrill.) 


LIFE    HISTORIES    OF   FUNGI  283 

buttons  enlarge,  a  little  chamber  forms  near  the  tip  (Fig, 
205),  and  into  this  chamber  some  of  the  hyphae  grow,  form- 
ing the  ''gills,"  with  their  ''fruiting"  surface.  While  the 
gills  are  forming  other  hyphas  form  a  veil,  extending  from 
the  stipe  to  the  edge  of  the  pileus,  and  protecting  the 
gills  until  the  spores  are  ripe.  By  continued  growth  of 
the  pileus  the  veil  becomes  ruptured,  thus  allowing  the 
spores  to  escape.  It  has  been  found  that  the  spores  do 
not  merely  fall  from  the  sterigmata  f rom  their  own  weight, 
but  that  they  are  forcibly  expelled. 

Entire  new  mushrooms  may  also  be  obtained  by  growing 
pieces  of  a  young  plant  on  suitable  nutrient  media.  From 
such  pieces  mycehum  is  produced  which  ultimately  bears 
the  "buttons." 

OUTLINE  OF  LIFE  HISTORY  OF  MEADOW-MUSHROOM 

Spore  (Basidiospore) 

4. 

Mycelium 

J- 

Binucleate  cells 

4.4. 

Young  sporophore  ("Buttons") 

Mature  sporophore  (the  "Mushroom")      \  ^!^^ 

Gills 

II 

Hymenium 

4.4, 

Basidium  {Nuclear  fusion) 

4,       \  Reduction 
Sterigma 

i 

Spore  (Basidiospore) 

272.  Other  Classes  of  Fleshy  Fungi.— In  addition  to 
the  gill-bearing  fungi  (Agaricaceae) ,  there  are  many  other 
families  of  Basidiomycetes,  one  bearing  the  fruiting  sur- 


284  STRUCTURE.  AND    LIFE    HISTORIES 

face  in  tubes  (Polyporaceae,  Fig.  206),  another  on  the  sur- 
face of  teeth  or  spines,  on  the  under  surface  of  the  pileus 
(Hydnaceae) .  The  common  "pufT-balls"  are  Basidio- 
mycetes,  with  the  fruiting  surface  entirely  enclosed  in 
the  more  or  less  globular  fruiting  body,  almost  the  entire 
contents  of  which  break  down  into  the  powder  or  dust  of 
the  ripe  puff-ball  (Fig.  207).  Over  14,000  species  of 
Basidiomycetes  have  been  described. 

273.  Life-cycle.- — The  life-cycle  of  the  meadow-mush- 
room (Agaricus  campestris)  may  be  indicated  as  shown  on 

page  283. 

OTHER  NON-GREEN  PLANTS 

In  addition  to  the  true  fungi,  there  are  two  other  groups 
of  non-green  thallophytes  which  ought  to  be  mentioned 
here,  one  (the  Myxomycetes)  because  of  their  scientific 
interest;  the  other  (bacteria)  because  of  their  economic 
importance. 

274.  Myxomycetes.— The  myxomycetes  are  on  the 
border-line  between  the  kingdom  of  plants  and  that  of 
animals.  In  some  of  their  characters  they  so  closely  re- 
semble lower  animals  like  the  Amceba,  that  they  have 
been  claimed  by  the  zoologists,  under  the  name  Mycetozoa 
(fungus-like  animals).  In  their  method  of  reproduction 
they  are  more  like  plants  than  like  animals. 

The  body  of  the  organism  is  a  large  naked  mass  of 
protoplasm,  called  a  Plasmodium,  commonly  found  on 
old  decaying  logs,  in  tan-bark,  and  similar  places,  where 
moisture  and  organic  food  are  abundant.  The  proto- 
plast flows  over  the  surface  on  which  it  grows,  like  the 
Amodba,  taking  in  nourishment,  at  times  flowing  around 
and  thus  injecting  particles  of  food,  assimilating,  and 
growing  in  size.     The  protoplasm  may  be  spread  out  in 


LIFE    HISTORIES    OF    FUNGI 


285 


sheets    or   in    a   delicate    network    of   strands,    or   both 
(Fig.    208).     When   viewed    under    the    microscope    the 


Fig.  208. — Plasmodium  of  a  slime-mold  (myxomycete),  Fuligo  sept  lea, 
growing  on  the  inner  surface  of  a  glass  jar.  The  natural  color  was  bright 
orange. 

protoplasm  is  seen  to  be  in  almost  constant  motion,  flow- 
ing for  about  a  minute  in  one  direction;  gradually  slowing 


Fig.  209. — Fruiting    bodies    of    a    myxomycete    {Comatricha    suksdorfii 
E.  and  E.),  greatly  enlarged.     (Photo  from  tyoe  specimen.) 

up,  it  comes  to  rest,  and  then  resumes  its  flowing,  but  in  the 
opposite  direction,  also  for  about  a  minute.     Different 


286 


STRUCTURE    AND    LIFE   HISTORIES 


species  possess  different  colors,  but  they  never  possess 
chlorophyll. 

275.  Reproduction. — At  a  certain  stage  of  development, 
the  Plasmodium  will  begin  to  form  tiny  upright  stalks, 
at  the  top  of  which  will  develop  a  spore-case,  containing 
spores  and  capillitium  (Fig.  209) .  The  capillitium  consists 
of  hygroscopic  threads  which  aid  in  the  dissemination  of 
the  spores.     When  the  spores  are  ripe  they  are  scattered 


Fig. 


-Kohlrabi,  showing  club-root,  caused  by  a  myxomycete, 
Plasmodia phor a  hrassiccB.     (Cf.  Fig.  2.) 


by  the  wind,  and  each,  on  germination,  produces  a  swarm- 
spore.  A  new  Plasmodium  results  from  the  fusion  of  these 
swarm-spores. 

276.  Economic  Importance. — The  Myxomycetes  have 
very  little  economic  importance,  but  the  disease  of  cab- 
bages, kohlrabi,  and  turnips,  known  as  club-root,  is  caused 
by  a  siime-mold  growing  on  the  roots  (Fig.  210). 

277.  Bacteria. — The    bacteria    are    among    the    very 


LIFE    HISTORIES    OF    FUNGI  287 

simplest  plant  structures  known.  They  are  one-celled, 
but  of  many  shapes,  and  with  or  without  motile  cilia. 
They  include  the  smallest  living  things  known.  There 
are  even  reasons  for  believing  that  some  forms  are  ultra- 
microscopic,  that  is,  too  small  to  be  seen  with  the  most 
powerful  microscopes  that  can  be  made.  Some  forms  are 
less  than  one  fifty-thousandth  of  an  inch  in  diameter. 
Of  some  kinds  of  bacteria,  as  many  as  300  could  be  placed 
side  by  side  on  the  period  at  the  end  of  this  sentence.  In 
fact  the  word  "microbe"  means  "tiny  living  thing, ''^ 
though  not  all  microbes  are  bacteria.  The  germ  of 
malaria,  for  example,  is  a  microscopic  animal  (protozoon), 
resembling  an  Amesba. 

Several  genera  of  bacteria  are  distinguished  according 
to  shape  as,  for  example,  Bacterium  (non-motile  rods), 
Bacillus  (motile  rods),  Micrococcus  (spherical).  Spirillum 
(spiral  threads)  (Fig.  211). 


Fig.  211. — Various  forms  of  bacteria,  a,  Spirillum;  b,  Bacillus  typhosus; 
c,  Staphylococcus;  d,  e,  j,  h,  Micrococcus;  f,  k,  I,  Bacillus;  g,  Pseudomonas 
Pycocyanea;  i,  Streptococcus. 

They  are  found  everywhere  and  multiply  rapidly  by 
cell-division,  whence  they  are  called  "fission-fungi,"  or 
Schizomycetes.  Not  possessing  chlorophyll,  they  are  all, 
of  course,  either  parasitic  or  saprophytic,  some  being 
highly  beneficial — in  fact  indispensable — to  man;  others 
highly  harmful,  causing  some  of  the  worst  known  diseases 
of  both  plants  and  animals. 

^  From  the  Greek  ixiKpSs  (mikros),  small,  +  /3/os  (bios),  life. 


CHAPTER  XX 

ECONOMIC  IMPORTANCE  OF  FUNGI 

In  the  preceding  chapter  we  have  frequently  referred 
to  ways  in  which  our  own  lives  are  related  to  the  life  of 
plants.  It  would  be  difficult  to  say  what  particular  group 
of  plants  affects  us  most,  but  certainly  none  more  than 
the  fungi.  They  are  among  the  direct  causes  of  human 
sorrow  and  happiness,  of  health  and  disease,  of  poverty 
and  wealth,  hfe  and  death.  They  are  at  once  the  founda- 
tion and  the  arch  enemy  of  agriculture,  the  objects  and  ob- 
stacles of  commerce,  the  source  and  hindrance  of  human 
industry. 

EDIBLE  FUNGI. 

278.  Mushrooms  and  Toadstools. — Everyone  is  familiar 
with  edible  ''mushrooms;"  they  are  now  on  sale  at  every 
grocery.  Nearly  everyone  thinks  there  is  a  difference 
between  mushrooms  and  ^'  toad-stools,"  Such,  however,  is 
not  the  case.  These  two  terms  are  applied  indiscrim- 
inately by  the  botanist  to  any  ''fleshy  "  fungus.  The  word 
"mushroom"  is  used  by  most  people  to  designate  the 
meadow-agaric  (Agaricus  campestris) ,  which  is  the  mush- 
room of  commerce,  par  excellence.  There  are  over  i,ooo 
fleshy  fungi  that  are  good  to  eat,  and  many  more  that 
are  not  poisonous,  but  non-edible  because  they  are 
tough,  or  fibrous,  or  ill-tasting.  Many  of  those  good 
to  eat  resemble  the  meadow-mushroom  in  having 
a  stalk  and  gills,  and  are  closely  related  to 
it    (Fig.    212);     while    others,    like    the    "puff-balls," 

288 


ECONOMIC    IMPORTANCE    OF    FUNGI  289 

belong  to  an  entirely  diiferent  group.  All  puff-balls  are 
good  to  eat  when  young.  Many  esteem  the  morel 
(Morchella  esculenta)  as  a  great  delicacy. 


Fig.  212. — Shaggy-mane  mushroom  (Coprinus  comatus).     Edible  before 
the  spores  turn  black. 

279.  Criterion  of  Edibility. — Here,  as  elsewhere,  there 
is  no  royal  road,  no  short  cut  to  knowledge.  There  are 
absolutely  no  external  characteristics  which  distinguish 
edible  from  poisonous  fungi.  The  only  way  to  tell  whether 
a  given  species  is  poisonous  or  not  is  to  try  it.  Since  this 
is  so,  it  is  best  for  the  amateur  not  to  make  the  endeavor, 
but  to  depend  only  upon  the  knowledge  of  an  experienced 
mycologist.  One  should  first  seek  to  attain  skill  in 
determining  the  exact  species  of  his  specimen,  and  then 
follow  the  assurance,  and  especially  the  warnings,  of 
some  reliable  book.  In  general,  one  should  avoid  all 
bright-colored  species  (although  some  of  them  are  not 
poisonous),  and  all  species  that  have  a  ''cup"  at  the 
base  of  the  stalk,  or  stipe.  To  insure  no  mistak^^  in  this 
latter  point,  one  should  always  be  sure  that  he  has  the 
base  of  the  stipe,  and  has  not  broken  it  off  above  the 
base.  Beginners  should  also  avoid  all  specimens  in  the 
*' button"  stage  of  development,  as  it  is  more  difficult 
to  determine  the  exact  species  at  that  stage. 
19 


290  STRUCTURE    AND    LIFE    HISTORIES 

280.  Mushroom  Culture. — The  growing  of  mushrooms 
for  the  market  is  a  very  important  industry,  especially 
in  some  localities.  Cultures  are  usually  started  from 
''spawn,"  obtained  from  the  seedsman  in  the  form  of 
''bricks."  These  bricks  consist  largely  of  mycelium, 
tightly  pressed  together.  When  the  bricks  are  broken 
up  and  distributed  through  a  "bed"  of  soil  and  manure, 
properly  prepared,  the  mycelium  resumes  its  growth,  and 
soon  begins  to  produce  the  "buttons"  (Fig.  204),  which 
finally  develop  into  mature  mushrooms. 

The  industry  is  commonly  carried  on  in  cellars  and 
caves.  This  is  not  necessary,  for  the  meadow-mushroom, 
as  its  name  clearly  implies,  grows  in  nature  in  open 
meadows  and  pastures.  But,  since  the  fungi  have  no 
chlorophyll,  they  do  not  need  the  light,  and  so  space 
can  be  used  for  their  culture  that  would  not  well  serve 
any  other  useful  purpose. 

FUNGI  THAT  CAUSE  PLANT  DISEASES 

281.  Govermnent  Regulation. — Fungi  that  grow  as 
parasites  on  green  plants  cause  serious  disturbances  of 
the  normal  life-processes  and  structure  of  their  hosts, 
interfering  with  healthy  growth,  and  causing  plant 
diseases.  Since  the  fungi  are  reproduced  by  spores,  these 
diseases  may  rapidly  spread  by  contagion.  On  this 
account  state  legislatures  and  the  national  Congress  have 
been  obliged  to  pass  stringent  laws  governing  international 
and  interstate  traffic  in  plants  liable  to  disease,  providing 
for  their  careful  inspection  and  quarantine.  The  United 
States  Government  maintains  an  expert  pathologist 
continuously  at  the  port  of  New  York  to  inspect  plants 


ECONOMIC   IMPORTANCE    OF    FUNGI  29 1 

imported  from  foreign  countries.  Sometimes  whole 
cargoes  of  potatoes  or  other  vegetables  are  refused  en- 
trance at  the  port,  and  must  then  be  taken  to  sea  and 
dumped  into  the  ocean,  or  else  taken  to  the  port  of  some 
other  country  where  the  regulations  are  less  stringent  or 
less  rigidly  inforced. 

282.  Diseases  Caused  by  Phycomycetes. — Among  the 
plant  diseases  caused  by  the  alga-like  fungi  may  be 
mentioned: 


Fig.  213. — "Little  potatoes."     A  disease  caused  by  the  parasitic  fungus, 
Rhizodonia  {Corticium  vagum  var.  solani  Burt). 

1.  The  ''damping-off  fungus"  {Pythium  de  Baryanum 
Hesse),  which  attacks  young  seedlings  of  beans  and  other 
plants  near  the  surface  of  the  ground,  causing  the  tissues 
there  to  disintegrate,  and  the  entire  plant  finally  to  wilt 
and  die. 

2.  Brown  rot  of  lemons,  commonly  seen  in  fruit  that 
has  been  kept  too  long  or  in  too  damp  a  place. 

3.  *' Blister-blight"  or  white  ''rust"  of  radishes  and 
their  relatives,  such  as   shepherd's  purse    and  mustard. 


292 


STRUCTURE    AND    LIFE    HISTORIES 


This  fungus  is  known  as  Albugo  Candida,  or  more  recentl)* 
as  Cystopiis  candidus. 

4.  Downy  mildew^  of  grapes  and  cucumbers. 


Fig.  214. — Witches'  brooms  on  the  hackberry  {Celtis  occidentalis), 
caused  by  a  gall-mite  {Phyioplus  sp.),  or  possibly  by  the  mite  in  conjunc- 
tion with  a  powdery  mildew  {Sphcerotheca  phytoptophyla),  which  is  usually 
found  on  the  "brooms." 

5.  Potato  rot  and  "late  blight,"  Phytophthora  injestans 
(Mont.)  DeBary.  This  disease  was  the  cause  of  the 
failure  of  the  potato  crop  and  the  consequent  famine  in 


ECONOMIC   IMPORTANCE    OF   FUNGI 


293 


Ireland,  in  1846-47.  (The  disease  known  as  "little  pota- 
toes" {Rhizoctonia)  and  by  various  other  names  (Fig.  213), 
is  caused  by  one  of  the  Basidiomycetes.) 

283.  Ascomycetes. — Among  diseases  caused  by  various 
species  of  sac-fungi  are  the  following: 


Fig.  215. — Ergot  {Claviceps  purpurea).  Sclerotia  on  wild  rye  {Elymus 
virginicus),  at  left;  marsh  grass  {Spartina  sp.),  middle;  cultivated  rye 
{Secale  cereale),  at  right. 

1.  Peach  leaf-curl,  plum  pockets,  and  "witches  brooms" 
(Fig.   214). 

2.  Brown  rot  of  peach  and  plum.  The  damage  caused 
by  this  disease  in  the  state  of  Georgia  alone,  in  1900, 
amounted  to  nearly  $700,000. 


294 


STRUCTURE    AND    LIFE   HISTORIES 


Fig    2i6  — Two  chestnut  trees  {Castanea  dentata),  killed  by  the  destructive 
chestnut  blight.     (After  W.  A.  Murrill.) 


ECONOMIC   IMPORTANCE    OF   FUNGI 


295 


3.  Alfalfa  leaf-spot,  the  sooty  mold  of  the  orange,  the 
powdery  mildew  of  grapes  and  apples,  the  wilt  disease  of 
cotton  and  watermelon,  the  ergot  of  rye  and  other  cereals 
(Fig.  215),  the  black  knot  of  plums  and  cherries,  and  the 
disastrous  chestnut  disease  of  the  eastern  United  States. 

284.  Chestnut  Disease. — The  chestnut  disease  (Fig.  216) 
first  appeared  in  the  vicinity  of  New  York  City  about 


Fig.  217. — Map  of  the  northeastern  United  States,  showing  the  approxi- 
mate distribution  of  the  chestnut  bark  disease  in  1911.  The  disease  has 
spread  further  since  the  map  was  made.  Horizontal  lines,  area  where 
majority  of  trees  are  dead;  vertical  lines,  approximate  area  where  infection 
is  complete;  dots,  location  of  advance  infections.  (After  Metcalf.  U.  S. 
Dept.  Agr.,  Farmers  Bull.  467.) 


1904,  and  from  there  as  a  center  it  has  rapidly  spread 
until  it  has  destroyed  most  of  the  chestnut  trees  within 
a  radius  of  150  to  200  miles  of  the  city  (Fig.  217).  As 
many  as  17,000  trees  have  been  destroyed  in  the  city  of 
Brooklyn  alone,  entailing  a  total  loss  of  several  million 


:^96 


STRUCTURE   AND    LIFE   HISTORIES 


dollars.  The  mycelium  of  the  fungus  that  causes  it 
(Endothia  parasitica)  grows  underneath  the  bark,  and 
for  this  reason  it  is   practically    impossible    to  check  it 


Fig.  2 1 8. — Chestnut  blight.  Portion  of  a  branch  of  an  American  chest- 
nut {Castanea  dentata),  which  had  been  artificially  inoculated  with  the 
spores  of  the  chestnut-blight  fungus,  Endothia  parasitica  (Murr.)  Anders. 
The  white  areas  are  infected  spots.     (After  W.  A.  Murrill.) 


by  spraying,  as  the  bark  protects  the  mycelium  from  all 
known  spraying  solutions.  The  fruiting  pustules  of  the 
fungus  form  on  the  surface  (Figs.   218  and  219).     The 


ECONOMIC   IMPORTANCE    OF   FUNGI 


297 


only  method  of  checking  the  spread  of  the  disease  is  to 
cut  down  and  burn  all  affected  trees.  Millions  of  dollars 
worth  of  damage  has  been  caused  by  this  disease  within 
the  past  seven  or  eight  years.  The  financial  loss  in  New 
York  City  and  vicinity,  alone,  has  been  estimated  at 
much  more  than  $5,000,000,  while  the  loss  for  the  entire 
United  States,  up  to  191 1,  was  estimated  by  the  Federal 


Fig.  219. — Chestnut-blight  fungus  {Endothia  parasitica).  Fruiting 
pustules  and  spore-masses  from  cultures.  X  about  8.  A,  stages  in  the 
development  of  the  pustules;  B,  C,  D,  various  forms  of  spore  discharge 
in  a  moist  atmosphere.     (After  Murrill.) 

Government  at  not  less  than  $25,000,000.  In  19 10,  the 
state  of  Pennsylvania  appointed  a  special  commission  of 
experts  for  the  investigation  and  control  of  the  disease, 
and  appropriated  over  $275,000  to  meet  the  necessary 
expense  of  the  work. 

286.  Grain  Smut. — The  smuts  of  wheat,  oats,  barley, 
and  corn  are  among  the  commoner  pests  of  the  farmer. 


298 


STRUCTURE    AND    LIFE    HISTORIES 


These  diseases  are  caused  by  fungi  of  the  genus  Ustilago} 
The  species  are  often  named  from  the  plant  affected, 
for  example   Ustilago  Avence  on  oats  (Avena,  Fig.   220), 


Fig.  220. — Panicles  of  oats  (Avena  saliva),  with  the  grains  almost  com- 
pletely destroyed  and  replaced  by  the  oat  smut  {Ustilago  Avence). 

Ustilago    Tritici    on  wheat  (Triticum),   Ustilago  Zece  on 
corn  (Zea,  Fig.  221).     The  mycelium  extends  through  the 

^  A  Hemibasidiomycete. 


ECONOMIC   IMPORTANCE    OF   FUNGI 


299 


Stem,  fruiting  in  the  tissues,  and  commonly  destroying 
the  kernel  of  grain.  The  innumerable  black  spores  form 
a  sooty  powder — whence  the  common  name  of  '^smut/' 


Fig.  221.— Corn-smut  {Ustilago  maydis)  on  stalk,  tassel,  ear,  and  leaf  of 

Zea  Mays. 

286.  Rusts.— The  life  history  of  the  wheat  rust  (Puc- 
cinia  graminis)  was  outlined"  in  Chapter  XIX.  This 
fungus  has  not  only  caused  millions  of  dollars  worth  of 
damage  to  the  wheat  crop  of  the  world  but  has  been  the 
cause  of  legislative  enactments.  As  early  as  1760  there 
was  passed  in  Massachusetts  ''An  Act  to  prevent  Damage 
to  English  Grain  arising  from  Barberry  Bushes."  This 
act  read,  in  part,  as  follows: 

"Whereas  it  has  been  found  by  experience,  that  the  Blasting  of  Wheat 
and  other  English  Grain  is  often  occasioned  by  Barberry  Bushes,  to  the 
great  loss  and  damage  of  the  inhabitants  of  this  province: 


300  STRUCTURE    AND    LIFE    HISTORIES 

"Be  it  therefore  enacted  by  the  Governour,  Council,  and  House  of 
Representatives,  that  whoever,  whether  community  or  private  person, 
hath  any  Barberry  Bushes  standing  or  growing  in  his  or  their  Land,  within 
any  of  the  Towns  in  this  Province,  he  or  they  shall  cause  the  same  to  be 
extirpated  or  destroyed  on  or  before  the  thirteenth  Day  of  June,  Anno 
Domini  One  Thousand  Seven  Hundred  and  Sixty. 

"Be  it  further  enacted  that  if  there  shall  be  any  Barberry  Bushes 
standing  or  growing  in  any  land  within  this  Province,  after  the  said  loth 
day  of  June,  it  shall  be  lawful,  by  Virtue  of  this  Act,  for  any  Person  who- 
soever to  enter  the  Lands  wherein  such  Barberry  Bushes  are,  first  giving 
one  month's  notice  of  his  intention  to  do  so  to  the  Owner  or  Occupant 
thereof,  and  to  cut  them  down,  or  pull  them  up  by  the  root,  and  then  to 
present  a  fair  account  of  his  labour  and  charge  therein  to  the  owner  or 
occupant  of  the  said  land;  and  if  such  owner  or  occupant  shall  neglect  or 
refuse  by  the  space  of  two  months  next  after  the  presenting  said  account, 
to  make  to  such  person  reasonable  payment  as  aforesaid,  then  the  person 
who  cut  down  or  pulled  up  such  bushes,  may  bring  the  action  against  such 
owner  or  occupant,  owners  or  occupants,  before  any  Justice  of  the  Peace, 
if  under  forty  shillings,  or  otherwise  before  the  Inferior  Court  of  Common 
Pleas  in  the  County  where  such  Bushes  grew,  who  upon  proof  of  the  cut- 
ting down  or  pulling  up  of  such  bushes  by  the  person  who  brings  the  action, 
or  such  as  were  employed  by  him,  shall  and  is  hereby  respectively  em- 
powered to  enter  up  judgment  for  him  to  recover  double  the  value  of  the 
reasonable  expense  and  labour  in  such  service  and  award  execution 
accordingly. 

"Be  it  further  enacted,  that  the  Surveyors  of  the  Highways,  whether 
public  or  private,  be  and  hereby  are  empowered  and  required  ex  officio  to 
destroy  and  extirpate  all  such  Barberry  Bushes  as  are  or  shall  be  in  the 
Highways  in  their  respective  Wards  or  Districts,  and  if  any  such  shall 
remain  after  the  aforesaid  tenth  Day  of  June,  Anno  Domini  One  Thou- 
sand Seven  Hundred  and  Sixty,  that  then  the  Town  or  District  in  which 
such  bushes  are  shall  pay  a  Fine  of  two  shillings  for  every  bush  standing  or 
growing  in  such  Highway,  to  be  recovered  by  Bill  Plaint,  Information,  or 
on  the  Presentment  of  a  Grand  Jury,  and  to  be  paid  one  Half  to  the  In- 
former and  the  other  Half  to  the  Treasury  of  the  County  in  which  such 
bushes  grew  for  the  use  of  the  County." 

In  addition  to  the  rust  of  wheat,  there  are  also  rusts  of 
the  carnation  and  the  clover  caused  by  species  of  Uro- 
myces.     The  carnation  rust,  which  first  appeared  about 


ECONOMIC  IMPORTANCE  OF  FUNGI         30I 

1892,  is  common  in  the  plant  houses  of  commercial 
florists. 

287.  Pine  Tree  Blister-rust. — Among  the  more  im- 
portant plant  diseases  recently  appearing  in  the  United 
States  is  the  pine  tree  blister-rust,  introduced  from 
Europe  about  1909.  One  species  is  Cronartium  pyri- 
forme,  which  is  the  telial  stage  of  Peridermium  pyriforme- 
The  aecial  stage  {Peridermium)  appears  on  the  pine, 
while  the  alternating  host  is  the  ''false  toad-flax"  {Com- 
andra  umbellata  and  C.  pallida).^  This  fungus  attack.* 
species  of  pine  that  have  less  than  five  leaves  to  the  fascicle, 
such  2is  Pinus  contorta,  P.  ponderosa,  and  P.  rigida. 

Another  species  (Cronartium  rihicola)  passes  its  aecial 
stage  on  five-leaved  pines,  where  it  is  commonly  known 
as  Peridermium  Strobi;^  the  telial  stage,  as  its  name  in- 
dicates, is  passed  on  species  of  Ribes  (gooseberries  and 
currants)  (Fig.  222). 

The  importance  of  such  a  disease  as  this  may  be  inferred 
when  we  consider  that  the  value  of  the  white  pine  grow- 
ing in  the  New  England  states  is  estimated  at  $75,000,000. 
that  of  the  Lake  states  at  $96,000,000,  of  the  Western 
states  at  $60,000,000,  and  of  other  National  forests  at 
$30,000,000,  a  total  of  $261,000,000.  The  western  sugar 
pine  {Pinus  Lambertiana)  has  a  total  value  estimated  at 
$150,000,000.  Thus,  timber  to  the  value  of  $411,000,000 
is  threatened  with  destruction  by  this  one  parasitic  dis- 
ease. In  order  to  reduce  the  danger  of  infection  from  the 
blister-rust,  and  also  from  the  pine-shoot  moth  {Evetria 

^  The  Comandra  is  itself  a  parasite  on  the  roots  of  various  species  of 
blueberry  (Vacciniiint),  and  other  woody  plants. 

2  From  Strohus,  the  specific  name  of  the  common  white  pine  {Pin*^ 
Strohus). 


302 


STRUCTURE    AND    LIFE    HISTORIES 


hiioliana),  the  United  States  Department  of  Agriculture 
has  issued  quarantine  regulations  forbidding  the  impor- 


FiG.  2  2  2. — White  pine  blister-rust.  A,  portion  of  diseased  tree,  show- 
ing pycnidial  blisters  broken  open;  from  these  blisters  the  disease  spreads 
to  neighboring  currant  or  gooseberry  bushes;  B,  early  summer  stage  on 
under  surface  of  a  currant  leaf;  these  spores  repeat  during  the  summer, 
at  intervals  of  two  weeks;  C,  early  summer  stage,  much  magnified;  D, 
late  summer  and  fall  stage,  on  the  under  surface  of  a  currant  leaf;  from 
this  stage  the  disease  spreads  again  to  pine  trees.  (After  Perley  Spaulding, 
by  courtesy  of  the  U.  S.  Dept.  of  Agriculture.) 

tation  of  all  five-leaved  pines  and  all  species  and  varieties 
of  Ribes,  except  for  experimental  or  scientific  purposes  by 


ECONOMIC   IMPORTANCE    OF    FUNGI 


303 


the  Department  of  Agriculture.  Since  July  i,  191 5,  the 
importation  of  every  spe- 
cies of  the  genus  Pinus  has 
been  forbidden  from  all 
European  countries  and  lo- 
calities. In  March,  19 16, 
the  Federal  Horticultural 
Board  requested  all  nursery- 
men in  the  eastern  United 
States  not  to  ship  white 
pine,  gooseberry  or  currant 
stock  into  the  Rocky  Moun- 
tain and  Western  white  pine 
forest  areas. 

288.  Timber-destroying 
Fungi. — Everyone  recalls 
the  "shelf -fungi,"  so  often 
seen  growing  on  the  trunks 
of  trees  (Fig.  223).  These 
forms  are  the  fruiting  bodies 
of  the  fungus,  while  the  my- 
celium ramifies  through  the 
wood,  often  in  such  quan- 
tities as  to  form  the  ''punk," 
formerly  much  used  in  set- 
ting off  fireworks.  The  soft 
fungal  threads  are  enabled 
to  make  their  way  through 
the  hard  woody  tissue  by 
means  of  an  enzyme  which 
they  secrete.  The  enzyme 
softens  and  dissolves   the   cell-walls   of   the   wood,  thus 


Fig.   223. — A  shelf-fungus  {Fomes 
applanatus)  on  sugar  maple. 


304 


STRUCTURE    AND    LIFE    HISTORIES 


making  it  possible  for  the  mycelium  to  penetrate  with  ease. 
This  process  disintegrates  the  wood,  weakens  the  tree  so  that 
it  eventually  dies  or  is  easily  blown  over  by  the  wind,  and 
of  course  renders  the  wood  of  little  or  no  value  for  timber. 
The  fungus  gains  admission  to  the  tree  by  means  of  the 
spores  falling  on  some  surface  freshly  exposed  by  trim- 
ming the  tree,  by  the  accidental  breaking  of  branches, 
by  the  ''barking"  caused  by  lawnmowers,  and  in  other 
ways;  on  account  of  the  disastrous  results,  all  such  sur- 
faces should  be  protected  by  be- 
ing painted  over  as  soon  as  a 
branch  is  cut  or  broken  off,  or  a 
portion  of  bark  removed.  There 
are  many  species  of  wood-de- 
stroying fungi,  and  the  financial 
loss  they  cause  to  the  lumber 
industry,  not  to  mention  the 
losses  of  beautiful  shade  trees 
in  lawns,  parks,  and  streets,  is 
very  considerable. 

MOLDS 

The  filamentous  fungi,  com- 
monly known  as  molds,  be- 
long to  various  species.  The 
black  mold  {Mucor  mucedo)  is 
common  on  bread,  and  the  blue 
mold  {Penicillium)  (Fig.  224) 
on  decaying  fruit  and  on  fruit  canned  at  home.  The 
appearance  of  the  mold  indicates  that  the  fruit  was  not 
sufficiently  sterilized  before  the  cover  was  screwed  down 
on  the  fruit  jar.     In  fact,  the  entire  process  of  canning  is  a 


Fig.  224. — Penicillium  glau- 
cum.  h,  hypha;  b,  basal  cell;  st 
sterigma;  c,  spore  {conidium). 


ECONOMIC  IMPORTANCE  OF  FUNGI         305 

series  of  operations  intended  primarily  to  kill  all  germs  of 
bacteria  and  fungi.  The  sterilized  fruit  is  then  sealed 
from  the  air  and  from  access  of  other  germs  while  it  is 
still  hot.  The  ''keeping"  of  canned  goods  depends  upon 
the  successful  exclusion  of  every  living  spore  or  other 
germ.  If  goods  preserved  in  tin  cans  have  been  im- 
perfectly sterilized  the  gases  produced  by  fermentation 
will  exert  a  pressure  upon  the  can  from  the  inside,  often 
strong  enough  to  cause  a  bulging  of  the  ends. 

COLD  STORAGE 

Just  as  canning  has  for  its  object  the  preservation  of 
vegetable  or  animal  tissues  by  killing  the  germs  with 
heat,  so  cold  storage  accomplishes  the  same  end  by  means 
of  extreme  cold.  Most  germs  remain  inactive  below  a 
certain  temperature,  which  may  be  readily  ascertained 
by  experiment.  The  spoiKng  of  eggs  is  caused  by  the 
presence  within  the  egg  of  a  germ-"  flora,"  which  lives 
upon  the  yolk  and  white,  producing  by  its  life  proc- 
esses, the  noxious  gases  of  stale  eggs.  At  certain  low 
temperatures  most  of  the  life-processes  of  the  germs 
are  either  stopped  entirely,  or  greatly  retarded.  In  the 
colonial  days  of  America,  and  later,  it  was  common  for 
dwellers  on  farms  and  in  villages  to  preserve  meat  by 
burying  a  quantity  of  it  in  the  snow  during  the  winter 
season — a  primitive  cold  storage.  During  cold  storage, 
however,  certain  chemical  changes  take  place  in  the 
preserved  tissue,  due  to  enzyme  action.  As  a  result 
there  is  a  limit  to  the  period  that  food  can  be  kept  in  cold 
storage  without  deteriorating.  All  cold-storage  plants 
and  refrigerator  cars  are  evidence  of  the  fact  that  our 
own  lives  are  profoundly  affected  by  the  existence  and 
activity  of  microscopic  forms  of  plant  life. 

30 


3o6  STRUCTURE    AND    LIFE   HISTORIES 

YEASTS 

We  are  all  familiar  with  "yeast/'  but  many  persons  do 
not  realize  that  yeast  is  a  plant,  and  that  there  are  vari- 
ous species.  The  baker's  yeast  is  different  from  the 
brewer's,  and  there  are  more  than  one  kind  of  the  latter — 
one  for  example  causing  "top  fermentation,"  another 
"bottom  fermentation."  There  are  also  various  "wild" 
yeasts,  present  in  the  air.  The  nature  of  fermentation 
has  been  discussed  in  Chapter  VIII.  It  is  interesting  to 
reflect  that  this  group  of  microscopic  plants,  in  its  relation 
to  bread  making  (from  one  kind  of  grain)  adds  to  the 
wealth  and  happiness  of  the  world,  and  ministers  to  one 
of  the  most  fundamental  needs  of  our  physical  being, 
while  in  its  relation  to  brewing  (the  formation  of  alcohol 
from  another  kind  of  grain)  it  contributes  to  one  of  the 
greatest  sources  of  poverty,  misery,  and  crime. 

The  use  of  yeast  in  bread  making  dates  from  prehistoric 
ages.  It  is  mentioned  in  old  testament  history  as  early 
as  the  patriarchal  age.  It  is  of  interest  also  to  reflect 
that  in  the  various  great  migrations  of  the  human  race, 
this  little  plant  must  have  been  preserved  and  transported 
with  as  much  care  as  domestic  animals.  In  such  a 
movement  as  the  colonization  of  a  new  continent,  as, 
for  example.  North  and  South  America,  and  Australia, 
provision  must  have  been  made  for  maintaining  the 
supply  of  yeast  for  the  making  of  bread. 

BACTERIA 

289.  Extent  of  Their  Influence. — The  study  of  bacteria, 
the  smallest  of  all  known  plants,  has  become  so  extensive 
as  to  form  a  separate  science,  bacteriology;  and  it  will  not 
be  possible  here  to  do  more  than  suggest  in  outline  the 


ECONOMIC  IMPORTANCE  OF  FUNGI 


307 


numerous  important  ways  in  which  these  tiny  plants  affect 
our  daily  lives.  In  fact  their  discovery  has  practically 
revolutionized    our    method    of    living    in    many    ways. 


Fig.  225. — Louis  Pasteur,  founder  of  the  science  of  bacteriology. 

Modern  methods  of  sanitation  and  hygiene,  public  and 
private,  modern  house  furnishing,  as,  for  example,  the 
substitution  of  bare  floors  with  rugs  in  place  of  carpets, 
modern  views  of  disease  and  methods  of  treating  it,  and 


3o8 


STRUCTURE    AND    LIFE    HISTORIES 


the  technique  of  innumerable  industries,  both  agricultural 
and  manufacturing,  have  all  been  profoundly  modified 


Fig.  226. — Tumors  on  a  black  walnut  {Juglans  nigra);  probably  crown 
galls  caused  by  bacteria. 

or  determined  altogether  by  the  discovery  of  bacteria, 
and  the  extension  of  our  knowledge  concerning  them. 


ECONOMIC  IMPORTANCE  OF  FUNGI        309 

290.  Bacteria  and  Plant  Diseases. — In  addition  to  the 
plant  diseases  caused  by  fungi,  as  mentioned  above,  a 
number  are  known  to  be  caused  by  bacteria.  The  "wilt" 
of  sweet  corn,  a  disease  first  discovered  on  Long  Island, 
is  caused  by  bacteria,  as  is  also  the  crown  gall,  a  tumorous 
or  cancerous-like  disease  common  in  the  rose  family 
(peaches,  apples,  roses,  raspberries),  and  the  walnut, 
grape,  and  willow  (Fig.  226).  The  "bean  blight "  and  pear 
blight,  the  soft  rot  of  the  calla-lily,  and  the  "wilt"  of 
cucumbers  and  melons,  are  also  caused  each  by  its  own 
peculiar  kind  of  bacterium.  On  account  of  their  nature 
these  diseases  may  all  be  transmitted  from  one  plant  to 
another  of  the  same  kind. 

291.  Contagious  and  Infectious  Diseases. — The  ease 
with  which  such  tiny  organisms  as  bacteria  can  be  trans- 
ferred from  one  place  to  another  makes  the  diseases  they 
cause  easily  transmissible  or  "catching."  We  actually 
do  "catch  cold";  that  is,  our  all  too  common  "colds"  are 
due  to  the  presence  of  "  cold  "-producing  germs.  Arctic 
explorers  testify  to  the  fact  that,  notwithstanding  the 
great  exposures  to  which  they  are  subjected,  they  never 
"catch  cold."  This  is  explained  by  the  absence  of  the 
"cold"  germs  that  cause  colds  in  other  climates  or 
regions. 

When  the  members  of  the  Peary  arctic  expedition  of 
1908-09  were  in  the  field  away  from  the  heat  and  infective 
dust  of  the  ship,  they  were  practically  immune  from  colds 
and  respiratory  troubles.  "The  fact  that  colds  are  due 
to  bacteria  was  clearly  demonstrated  in  the  Arctic. 
We  might  be  precipitated  into  icy  water  with  the  air  many 
degrees  below  zero;  our  clothing  saturated  with  moisture 


3IO  STRUCTURE   AND  LIFE   HISTORIES 

on  the  march;  yet  we  could  sleep  in  our  damp  garments 
without  fear  of  taking  cold/'^ 

292.  Immimity. — This  is  not  the  place  to  discuss  the 
various  theories  of  immunity,  but  the  fact  should  be  noted 
in  passing.  It  is  a  common  belief  that  one  who  has  had 
the  measles  or  the  mumps  cannot  have  the  same  disease 
again.  While  this  is  extremely  doubtful,  it  is  known  that 
once  having  a  disease  does  render  one  less  liable  to  con- 
tract it  a  second  time.  When  the  germs  multiply  in  the 
body  with  the  first  attack,  the  toxin  they  produce  stimulates 
the  various  cells  to  secrete  an  antitoxin,  which  counteracts 
the  toxin,  or  poison.  Some  persons  appear  to  be  naturally 
immune  to  certain  diseases  {e.g.,  hay  fever),  while  others 
are  specially  susceptible. 

293.  Disease  Carriers. — Persons  who  are  immune  may, 
however,  unknowingly  transmit  the  disease-germ  from 
one  person  to  another.  They  are  called  ''carriers." 
Typhoid  fever,  caused  by  Bacillus  typhosus,  is  often  trans- 
mitted by  "typhoid  carriers,"  a  recent  case  being  that 
of  "Typhoid  Mary,"  a  domestic  servant  near  New  York 
City,  who  for  several  years  endangered  the  lives  of  others 
in  homes  and  hospitals  where  she  was  employed.  The 
menace  of  such  persons  to  public  health  justifies  their 
permanent  isolation. 

294.  Combating  Disease. — There  are  two  ways  of 
combating  disease  in  either  plants  or  animals:  (i)  to 
guard  against  it  in  advance  {prophylaxis),  thereby  en- 
deavoring to  prevent  its  appearance;  (2)  to  treat  it  after 
it  has  appeared.     Obviously  the  former  is  the  more  im- 

^  From  a  letter  to  the  author  from  John  W.  Goodsell,  M.  D.,  Surgeon 
of  the  Peary  arctic  expedition  of  1908-09. 


ECONOMIC   IMPORTANCE    OF   FUNGI  31I 

portant  and  effective.     The  most  effective  prophylactic 
measures  are  the  following: 

1.  Personal  hygiene ^  involving  bodily  cleanliness,  tem- 
perate habits,  and  careful  diet.  With  plant  diseases 
hygienic  measures  include  such  practices  as  sterilizing 
seeds  before  planting,  by  rinsing  them  in  solutions  of 
some  germ-killing  substance,  like  formaldehyde;  spray- 
ing diseased  trees  with  fungicides  (fungus-kilhng  solu- 
tions) ;  washing  the  branches  and  foliage  with  various  solu- 
tions, such  as  whale-oil  soap  (good  for  scale  insects) ;  and 
painting  the  cut  surfaces  of  trees,  after  trimming,  to 
prevent  the  mycelium  of  germinating  fungus  spores  from 
entering  the  woody  tissue. 

2.  Public  hygiene,  or  sanitation,  which  means  maintain- 
ing healthful  surroundings.  In  the  case  of  am'mal  dis- 
eases this  includes  preserving  a  pure  pubHc  water  supply, 
proper  sewage  systems,  clean  streets,  a  pure  milk  supply 
(especially  a  healthful  condition  of  the  cows  and  their 
surroundings),  and  a  careful  inspection  of  meat  and  all 
other  foods  shipped  and  sold  in  public. 

In  the  case  of  plants,  sanitation  includes  preserving 
a  proper  drainage  and  aeration  of  the  soil;  maintaining  a 
pure  atmosphere,  and  especially  one  free  from  smoke  and 
the  poisonous  gases  that  accompany  it;  fumigating  in 
plant  houses  with  potassium  cyanide  fumes,  or  with  to- 
bacco smoke  to  kill  scale  insects,  and  other  insects,  the 
burning  up  of  trees  or  other  plants  infected  with  a  trans- 
missible disease,  such  for  example,  as  the  chestnut  bark 
disease,  or  barberry  bushes  carrying  wheat  rust,  and 
eradication  of  injurious  fungi  from  the  soil  of  cultivated 
fields  by  crop-rotation,  as  indicated  in  Chapter  VII. 

3.  Quarantine.     This  is  a  method  of  sanitation  by  which 


312 


STRUCTURE    AND    LIFE    HISTORIES 


diseased  individuals  are  prevented  by  isolation  from  com- 
ing into  contact  with  those  who  are  well.  When  one 
member  of  the  family  is  sick  with  a  contagious  disease  he 


Fig.  227. — Map,  illustrating  the  inter-continental  migration  of  plant 
diseases.  No.  i,  potato  blight:  Chili-Colorado-Europe.  No.  2,  asparagus 
rust:  Europe,  1805;  New  Jersey,  1896;  South  Carolina,  1897;  Michigan, 
1898;  Illinois,  1899;  Dakota,  Nebraska  and  Texas,  1900;  California,  1901. 
No.  3,  potato  cercosporose:  Europe,  1854;  United  States,  1903.  No.  4, 
rice  smut:  Japan-South  Carolina,  1898.  No.  5,  sorghum  smut:  Japan- 
United  States,  1884.  No.  6,  grape  anthracnose:  Europe- America,  1880, 
or  earlier,  now  widespread.  No.  7,  cucumber  downy  mildew:  Cuba,  1868; 
United  States,  1889.  No.  8,  grape  black  rot:  N)rth  America,  early; 
France,  1885;  Italy  and  the  Caucasus,  1898.  No.  9,  potato  vvart:  Hun- 
gary, 1896;  England,  1900;  Newfoundland,  1909;  Boston  and  New  York, 
1910.  No.  10,  grape  downy  mildew:  America  early;  France,  1873;  the 
Rhineland,  Savoy  and  Italy,  1879;  The  Tyrol  and  Algiers,  1880;  Portugal 
and  Greece,  1881;  Alsace,  1882;  the  Caucasus,  1887;  Brazil,  1890.  Now 
known  in  all  countries  except  Australia.  No.  11,  grape  powdery  mildew: 
United  States,  early;  England,  1845;  Belgium  and  France,  1848;  all  Europe 
1849;  Madeira,  1852.  Known  everywhere  now.  No.  12,  chrysanthemum 
rust:  Japan-England,  1895;  America,  1896.     (After  F.  L.  Stevens.) 


should  be  confined  to  one  part  of  the  house,  apart  from 
the  others,  and  not  allowed  to  mingle  with  them  until  well. 
Hospitals  have  *' isolation  wards"  where  persons  with 
communicable  diseases  are  kept  apart  from  other  patients. 


ECONOMIC  IMPORTANCE  OF  FUNGI         313 

Immigrants  coming  to  America  from  foreign  countries  are 
carefully  examined,  and  if  they  have  a  contagious  disease 
they  are  isolated  (placed  in  quarantine)  until  well. 

The  United  States  Government  maintains  a  stringent 
quarantine  against  the  shipment  of  diseased  plants  from 
one  state  to  another,  or  from  foreign  countries  into  the 
United  States.  Some,  if  not  all,  of  our  worst  plant  diseases 
have  been  imported.  The  map  (Fig.  227)  shows  how  the 
rice  smut  travelled  from  Japan  to  South  CaroHna  in  1898, 
the  chrysanthemum  rust  from  Japan  through  England 
(1895)  t^  America  (1896),  and  the  potato  blight  from  Chili 
to  Colorado  and  across  North  America  to  Europe  (1845). 
The  downy  mildew  of  the  grape  is  an  example  of  a  disease 
probably  originating  in  North  America,  where  it  has  been 
known  from  the  earliest  times,  and  travelling  thence  to 
France  (1873)  ^-nd  other  parts  of  Europe,  reaching  as  far 
as  Greece  by  1881,  and  to  Brazil  by  1890. 

These  brief  references  indicate  the  importance  of  main- 
taining a  strict  quarantine  on  plants  at  all  our  ports  of 
entry. 

4.  Breeding  of  resistant  varieties.  This  is  one  of  the 
most  important  of  all  prophylactic  measures.  Just  as 
some  persons  are  immune  to  certain  contagious  diseases, 
so  certain  plants  in  a  crop  are  less  susceptible  than  others, 
or  even  entirely  immune,  to  a  given  disease.  By  choosing 
seed  each  year  only  from  the  immune  or  most  resistant 
individuals,  a  crop  may  sometimes  be  obtained  which  not 
only  withstands  the  disease  itself,  but  interferes  with  or 
finally  stops  entirely  its  spread.  No  phase  of  plant  breed- 
ing is  more  important  than  this. 

5.  Vaccination.  When  bacteria  produce  poisons  (/oa:^^) 
in  the  system,  the  cells  affected  are  stimulated  to  secrete 


314  STRUCTURE    AND    LIFE    HISTORIES 

an  antitoxin  which  counteracts  the  influence  of  the  toxin 
(Cf.  p.  310).  The  production  of  these  antitoxins  may 
finally  completely  nullify  the  effect  of  the  toxin,  and  then 
the  patient  ''gets  well."  The  presence  of  the  antitoxin, 
thus  produced,  explains  why  one  who  has  recently 
recovered  from  a  contagious  disease,  like  measles,  or 
mumps,  or  whooping  cough,  is  more  or  less  immune  for 
a  longer  or  shorter  period.  In  1796  the  English  physician 
Jenner  observed  that  persons  who  had  cowpox,  a  mild 
form  of  smallpox,  were  commonly  immune  to  the  latter. 
Reasoning  from  this  he  developed  the  method  of  vaccina- 
tion. By  this  method  the  cowpox  is  first  given  to  a  calf 
or  a  heifer,  or  sometimes  to  an  adult  cow.  At  the  end 
of  five  to  seven  days  pustules  occur  on  the  infected  surface 
of  the  animal.  A  watery  substance  within  these  pustules 
is  then  collected  by  sterile  instruments  and  carefully 
tested  to  make  sure  that  it  does  not  contain  any  germs 
of  tuberculosis  or  other  disease.  This  substance  is  the 
vaccine,  and  in  vaccination  a  small  portion  of  it  is  applied 
to  a  scratched  or  slightly  lacerated  area  on  a  person's 
arm.  A  mild  form  of  the  disease  results,  causing  the 
formation  of  an  antitoxin  in  the  person's  blood,  and 
thus  rendering  him  actively  immune.  The  word  vaccina- 
tion is  derived  from  the  Latin  word  vacca  (a  cow) ,  in  allu- 
sion to  the  method  of  obtaining  the  vaccine.  It  has  been 
calculated  that,  in  large  armies,  fully  as  many  lives  have 
been  saved  from  disease  by  vaccination  against  typhoid, 
cholera,  and  other  diseases  as  are  lost  in  battle. 

6.  Serum-therapy.  The  treatment  of  germ  diseases 
by  serum-therapy  consists  in  injecting  into  the  blood  of 
the  patient  an  antitoxin,  specific  for  the  disease  to  be 
treated.     The  antitoxin  is  contained  in  the  blood-serum 


ECONOMIC    IMPORTANCE    OF   FUNGI  315 

of  some  animal  that  has  been  rendered  immune  to  the 
disease.  As  in  the  case  of  the  preparation  of  vaccine,  the 
animal  is  first  placed  in  quarantine,  under  the  most  perfect 
sanitary  surroundings;  if  found  free  from  all  contagious 
diseases,  and  otherwise  satisfactory,  he  is  given  increas- 
ingly large  doses  of  the  toxin  or  the  virus  that  causes  the 
given  malady.  This  treatment  may  require  as  long  as 
six  weeks,  and  results  in  the  formation  of  quantities  of  the 
antitoxin  in  the  blood.  A  quantity  of  blood  is  then  drawn 
from  the  animal,  and  the  blood-serum  isolated,  filtered, 
carefully  tested  for  purity,  content  of  antitoxin,  and  free- 
dom from  disease-germs,  and  finally  put  up  in  glass 
syringe  containers  ready  for  use.  When  a  person  is 
exposed  to  the  given  disease  {e.g.,  diphtheria),  or  has 
actually  contracted  it,  the  serum  is  injected  into  his 
circulatory  system,  where  the  antitoxin  counteracts  the 
toxin  of  the  disease.  The  patient  is  thus  rendered 
passively^  immune.  Serum-therapy  is  now  successfully 
employed  in  the  treatment  of  diphtheria,  tetanus  (lock- 
jaw), hog  cholera,  and,  with  more  or  less  success,  of 
infantile  paralysis  and  certain  other  diseases. 

Nothing  corresponding  to  vaccination  and  serum  therapy 
is  known  for  a  certainty  in  the  treatment  of  plant  diseases. 

7.  Antiseptic  surgery.  The  greatest  obstacle  to  suc- 
cessful surgery  has  always  been  the  presence  of  the  rich 
and  varied  microscopic  flora,  or  plant  life,  in  the  air. 
When  a  wound  was  opened  or  a  cut  made  the  germs  com- 
posing this  flora  found  on  the  cut  surface  the  most  favor- 
able conditions  for  their  growth  and  multiplication,  and 
the  poisons  they  secreted  interfered  with  the  healing  of 

^  Passively,  because  the  antitoxin  is  not  produced  by  the  activity  of  his 
own  cells,  as  it  is  in  the  case  of  vaccination. 


3l6  STRUCTURE    AND    LIFE   HISTORIES 

the  wound  and  caused  gangrene,  or  inflammations,  more 
fatal  than  the  disease  for  which  the  operation  was  under- 
taken. When  the  discovery  of  bacteria,  and  of  their 
universal  presence,  enabled  men  to  understand  these 
facts  it  was  only  a  step  to  the  practice  of  making  the 
operating  rooms,  the  surgical  instruments,  and  the  surface 
to  be  cut,  absolutely  aseptic  or  sterile,  thus  making  possible 
the  almost  unbelievable  achievements  of  antiseptic  surgery. 
Only  a  step!  But  what  a  wonderful  and  all-important 
step  for  the  human  mind  to  take.  The  honor  and  credit 
for  taking  it  belong  chiefly  to  the  famous  English  surgeon, 
Lord  Lister. 

HELPFUL  BACTERIA 

295.  Bacteria  and  the  Dairy. — Not  all  bacteria,  by 
any  means,  are  harmful  to  man.  Many  of  the  practices 
of  the  dairy,  for  example,  are  dependent  upon  the  action 
of  bacteria.  This,  in  part,  is  thought  to  explain  the 
peculiarly  delicious  flavor  of  June  butter.  The  souring 
of  milk  is  caused  by  the  action  of  substances  produced 
by  the  bacteria  present  in  all  unsterilized  milk.  In 
fact  the  familiar  flavor  of  milk  is  due  in  large  measure 
to  the  presence  in  it  of  certain  bacteria  and  the  sub- 
stances they  produce.  When  milk  is  obtained  under 
strictly  sanitary  conditions  ("certified"  milk)  it  loses  much, 
if  not  all,  of  its  characteristic  and  familiar  flavor.  The 
ripening,  flavoring,  and  other  peculiarities  of  the  various 
varieties  of  cheese  are  due  in  part  to  the  fact  that  they 
ripen  under  the  influence  of  different  kinds  of  bacteria 
or  molds.  The  '^ ripening"  is  caused,  in  part,  by  the 
action  on  the  substance  of  the  cheese  of  the  enzymes 
peculiar  to  the  various  fungi  or  bacteria  growing  in  it. 


ECONOMIC    IMPORTANCE    OF    FUNGI 


317 


and  in  part  by  the  action  of  an  enzyme  {galadase)  inherent 
in  the  milk  from  which  the  cheese  is  made. 

296.  Vinegar. — Cider  vinegar  is  in  every  respect  a 
plant  product.  It  is  formed  from  the  juice  of  apples,  and 
ripens  or  ''ages"  under  the  influence  of  bacteria.  These 
bacteria  produce  an  enzyme  which  causes  ''acetic  acid 


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Fig.  228. — Roots  of  soy  bean  plants.  A,  from  seeds  that  were  soaked 
in  soy  bean  bacterial  culture  over  night;  B,  roots  of  check  plant  (not  inocu- 
lated). (After  Garman  and  Didlake.  Courtesy  of  Kentucky  Agr.  Exp. 
Station.) 

fermentation."  This  is  the  acid  to  which  vinegar  owes 
its  sourness.  There  is  not  room  here  to  explain  the 
process  of  vinegar  making  in  detail. 

297.  The  Secret  of  Clover. — Farmers  have  known  for 
many  years  that  a  given  piece  of  land  produces  a  larger 
yield  if  the    ame  crop  is  not  grown  continuously,  season 


3lS  STRUCTURE    AND    LIFE    HISTORIES 

after  season,  but  if  there  is  an  alternation  or  rotation  oj 
crops  of  different  kinds.  It  was  also  recognized  that  the 
best  results  are  obtained  when  one  of  the  crops  in  rota- 
tion is  a  legume  (a  member  of  the  family  Leguminosae) 
such  as  clover,  peas,  beans,  vetch,  lentils,  and  others.-^ 
A  clear  explanation  of  the  value  of  rotation  was  not 
possible  until,  in  1889,  Hellriegel  discovered  that  legumes 
are  able  to  utilize  the  free  nitrogen  of  the  air,  and  that 
this  is  made  possible  by  the  bacteria  that  produce  the 
characteristic  tubercles  on  their  roots.  The  researches 
of  Hellriegel  and  others,  at  about  this  time,  proved  that 
the  fixation  of  nitrogen  is  due  to  the  bacterium  that 
causes  the  tubercles,  Pseudomonas  radicicola  (Fig.  61).  If 
clover  or  other  legumes  are  grown  in  a  sterile  soil,  free 
from  the  presence  of  all  bacteria,  the  tubercles  do  not 
form  (Fig.  228),  and  the  fixation  of  free  nitrogen  ceases. 
Thus  it  is  seen  that  bacteria  are  essential  to  one  of  the 
oldest  and  most  fundamental  practices  of  agriculture. 

289.  Nitrifying  Bacteria  in  the  Soil. — In  addition  to 
the  symhionts  causing  leguminous  tubercles,  there  exist, 
in  all  soil,  at  least  two  other  forms  of  nitrifying  bacteria, 
which  grow  independently  of  other  plants.  The  first,  by 
the  addition  of  oxygen,  transforms  the  ammonia  in  the 
soil  into  nitrites,  while  the  second,  by  the  addition  of 
more  oxygen,  transform  nitrites  into  nitrates.'^  It  is  only 
in  the  latter  form  that  nitrogen,  so  indispensible  to 
nutrition,  can  be  utilized  at  all  by  plants. 

299.  The  Nitrogen  Cycle.— From  the  above  facts  it 
is  seen  that  there  is,  in  nature,  a  nitrogen  cycle  quite  as 

^  The  subject  of  rotation  of  crops  is  more  fully  discussed  in  para- 
graph 90  (pp.  91-93). 

2  Cf.  Chapter  VII,  pp.  82-83. 


ECONOMIC  IMPORTANCE  OF  FUNGI 


319 


remarkable  as  the  carbon  cycle.  By  respiration  and 
burning,  complex  compounds  of  carbon  (carbohydrates 
and  hydrocarbons)  are  broken  down  and  the  carbon  re- 
leased, either  as  pure  carbon  (C)  or  as  the  simple  com- 
pound, carbon  dioxide  (CO2),  which  is  taken  in  by  green 
plants  and  recombined  into  the  complex  carbohydrates 
by  the  process  of  photosynthesis  (page  77). 


n\ 


.  Nitrate   -. 
■.'Bacteria  ■.•■.. 
(Nitroliacter)^ 


.;-  Nitrite  ;•• 
■.".'•"  Bacteria  • 
(NitroBOraonae); 


(Nitrites] 


Denitrifying 
.  .■  Bacteria  ••.• 


[Nitrogen^ 


Fig.  229. — The  nitrogen  cycle. 

So  also,  by  the  action  of  proteolytic  (protein  dissolving) 
enzymes  produced  by  bacteria,  certain  complex  com- 
pounds of  nitrogen,  the  proteins,  are  broken  down,  by 
putrefaction  and  similar  processes,  into  simpler  com- 
pounds, such  as  ammonia  (NH3),  or  still  further  dis- 
integrated until  free  nitrogen  results.     Were  it  not  for 


320  SlRUCTURE    AND    LIFE    HISTORIES 

the  nitrogen-fixing  bacteria  this  process  would  continue 
until  all  complex  nitrogenous  compounds  were  reduced 
to  ammonia  or  free  nitrogen.  All  life  would  then  cease, 
because  green  plants,  upon  which  animal  life  is  de- 
pendent, are  unable  to  live  without  nitrogen,  and  they 
cannot  utilize  it  in  the  form  of  ammonia  nor  of  free 
nitrogen.  Without  chlorophyll  and  the  nitrogen- fixing 
bacteria,  therefore,  all  life  would  cease.  Thus  it  is  that 
life  and  death,  health  and  disease,  wealth  and  poverty, 
misery  and  happiness,  are  dependent  upon  the  activity 
of  tiny  plants,  too  small  to  be  seen,  except  under  the 
highest  powers  of  the  microscope. 


CHAPTER  XXI 
SAPROPHYTISM  AND  SYMBIOSIS 

300.  All  Food  Organic. — In  more  ways  than  one  it  is 
true  that  life  is  dependent  upon  antecedent  life.  An 
illustration  of  this  principle  is  seen  in  the  case  of  nutri- 
tion, for  no  Kving  thing,  neither  plant  nor  animal,  can 
utilize  directly  as  food  the  unaltered  inorganic  elements 
and  compounds  derived  from  the  air  and  soil.  This  was 
clearly  shown  in  Chapter  VII.  While  plants  are  com- 
monly said  to  obtain  their  food  from  the  air  and  soil, 
strictly  speaking  this  is  not  true.  Carbon,  oxygen, 
hydrogen,  phosphates,  nitrates,  sulphates,  et  cetera  are 
no  more  plant  food  than  are  flour,  water,  sugar,  and  salt 
bread.  Just  as  the  elements  composing  bread  would, 
if  eaten  separately,  make  a  very  unsavory  and  poor  diet, 
so  the  inorganic  elements  and  compounds,  as  such,  would 
not  be  able  at  all  to  nourish  plants.  They  must  first  be 
broken  down  (if  compounds),  and  then  recombined  into 
the  organic  compounds  of  carbohydrates,  proteins,  and 
fats. 

301. — ^Necessity  for  Chlorophyll. — As  we  have  seen  in 
Chapter  VII,  this  recombination  of  inorganic  chemical 
elements  into  organic  compounds  is  the  function  of 
chlorophyll.  Thus  it  is  that  green  plants  are  absolutely 
essential  to  all  life,  and  as  we  learned  in  Chapter  XIX, 
this  explains  why  all  non-green  plants  are  found  only  in 
intimate  association,  either  with  living  green  plants  or 
with  the  organic  remains  or  products  of  other  living 
91  321 


32  2  STRUCTURE    AND    LIFE    HISTORIES 

things — plants  or  animals.     Not  being  able   to  manu- 
facture their  own  food,  they  must  find  it  ready  made. 

SAPROPHYTISM 

302.  Decay. — Perhaps  the  simplest  case  of  the  nutrition 
of  non-green  plants  is  the  absorption  of  food  from  the 
organic  remains  of  other  plants  or  of  animals.  When 
the  spores  or  other  reproductive  bodies  of  such  plants 
begin  to  grow  upon  such  a  substratum,  they  secrete 
various  enzymes  which  begin  to  disintegrate  it,  reducing 
it  to  simpler,  soluble  substances.  This  is  the  process 
commonly  known  as  *' decay,"  and  the  plants  which 
cause  it  are  called  saprophytes}  The  word  ''decay" 
is  derived  from  a  Latin  word,  decidere,  which  means 
to  fall  apart,  in  allusion  to  the  fact  that  the  decaying 
substance  is  being  disintegrated  or  broken  down  into 
simpler  substances,  which  are  recombined  by  assim- 
ilation, in  the  cells  of  the  saprophyte,  into  protoplasm 
like  its  own.  Such  a  state  of  existence  is  called  sapro- 
phytism. 

303.  Fungus -saprophytes. — Among  the  more  familiar 
saprophytic  plants  may  be  mentioned  the  common  bread- 
mold,  the  fungi  that  are  instrumental  in  ripening  cheese, 
the  so-called  ''mildews,"  which  often  grow  on  old  moist 
pieces  of  leather,  and  numerous  other  filamentous  fungi; 
the  bacteria  which  cause  the  decay  of  meat  and  other 
substances,  bacteria  which  cause  the  retting  of  flax  stems, 
thus  freeing  the  bast  fibers  from  which  linen  is  made  by 
causing  the  decay  or  rotting  away  of  the  remainder  of 
the  tissue,   the  bacteria  which  convert  cabbage  leaves 

^  From  the  Greek  words  sapros,  rotten  +  phyton,  plant. 


SAPROPHYTISM   AND    SYMBIOSIS  323 

into  sauerkraut,  the  numerous  slime-molds  (Myxo- 
mycetes),  and  various  other  forms,  which  are  more  or 
less  intimately  connected  with  human  industries,  or  with 
public  or  private  hygiene. 

Humus,  one  of  the  most  important  compounds  of  soil, 
is  composed  almost  entirely  of  the  remains  ot  plant 
and   animal  bodies  in   various  stages  of  disintegration, 


Fig.  230. — Indian    pipe    {Monotropa    uniflora).     (Photo    by    Elsie    M. 

Kittredge.) 

and  the  disintegration  is  caused,  almost  entirely,  by  the 
action  of  enzymes  secreted  by  bacterial  and  fungal 
saprophytes.  Thus  we  come  to  reahze  that  these  sapro- 
phytic plants  are  absolutely  essential  to  the  most  funda- 
mental of  all  human  occupations,  namely  agriculture. 

304.  Saprophytic  Flowering  Plants. — Not    all    sapro- 
phytes are  fungi.     The  Indian-pipe  {Monotropa  uniflora^ 


324  STRUCTURE    AND    LIFE   HISTORIES 

Fig.  230),  and  its  close  relative,  the  false  beech-drops 
(Monotropa  Hypopitys,  Fig.  231),  are  examples  of  flowering 
plants,  wholly  devoid  of  chlorophyll,  and  therefore  unable 
to  manufacture  their  food,  which  is  absorbed  entirely 
from  the  humus  in  which  they  grow.  Other  examples 
are  Lathrcca,  and  the  coral-root  {Corallorhiza). 


Fig.  231. — False  beech  drops  {Monotropa  Hypopitys).     (Photo  by  Elsie 
M.  Kittredge.) 


SYMBIOSIS 

305.  Different  Kinds  of  Symbiosis. — The  absorption  of 
nourishment  from  one  plant  by  another  involves,  of  course, 
the  intimate  association  of  the  two  organisms.  Such  a 
vital  association  is  called  symbiosis  (living  together),  and 
we  find  organisms  living  together  in  all  degrees  of  intimacy 


SAPROPHYTISM   AND    SYMBIOSIS  325 

and  independence,  or  of  interdependence.  One  plant 
may  merely  live  upon  another,  without  deriving  any 
nourishment  from  it  {epiphytism)  \  or  two  plants  may 
be  mutually  helpful,  each  contributing  something  of 
advantage  to  the  other  {mutualism)',  one  plant  may 
live  at  the  expense  of  the  other,  deriving  nourishment 
from  it,  but  contributing  little  or  nothing  in  return  {para- 
sitism); or  the  two  organisms  may  maintain  a  loose  or 
disjunctive  symbiosis,  which  may  be  either  (i)  nutritive, 
as  in  those  cases  where  ants  cultivate  filamentous  fungi, 
maintaining  fungus-farms;  or  (2)  non-nutritive,  as  in  the 
cases  where  certain  plants  like  clover  or  orchids,  are  de- 
pendent upon  insects  for  the  transfer  of  pollen  from  one 
flower  to  another.  These  phases  of  symbiosis  are  indi- 
cated in  the  following  table: 


Symbiosis 


1.  Disjunctive  or  "social." 

{a)  Nutritive   {e.g.,  ants  and  fungus-farms). 
{b)  Non-nutritive  (insects  and  pollination). 

2.  Epiphytism. 

3.  Mutualism. 

4.  Parasitism. 


306.  Social  Symbiosis.— As  an  illustration  of  social 
symbiosis  of  a  nutritive  character  may  be  mentioned  the 
interesting  relation  established  between  certain  leaf- 
cutting  ants  and  a  filamentous  fungus.  The  ants  remove 
the  foHage-leaves  from  certain  trees  and  use  them  as 
''fungus-farms,"  or  a  suitable  substratum  on  which  to 
cultivate  a  certain  fungus,  portions  of  which  serve  as 
food  for  the  ants  (Fig.  232).  The  spores  are  sown  by  the 
ants  and  the  ''crop"  harvested  in  a  very  systematic  man- 
ner. The  loss  of  leaves,  however,  is  very  deleterious  to 
the  life  of  the  tree,  and  certain  species  {e.g.,  Cecropia  and 


326 


STRUCTURE    AND    LIFE    HISTORIES 


Fig.  232. — Diagram  of  a  nest  of  a  fungus-growing  ant  {Trachymyrmex 
obscurior),  showing  four  chambers.  The  pendant  white  masses  in  three 
of  the  chambers  are  the  mycelium  of  the  fungus — the  so-called  "fungus- 
gardens."  The  species  of  the  fungus  has  not  been  definitely  determined, 
but  they  are  thought  to  belong  to  the  Ascomycetes.  (After  W.  M. 
Wheeler.) 


Fig.  233.— Epiphytes  or  "air"  plants  {Tillandsia  sp.),  growing  on  tele- 
phone wires  at  Ponce,  P.  R, 


SAPROPHYTISM   AND    SYMBIOSIS  327 

Acacia),  secrete  a  substance  which  is  greatly  liked  by 
another  kind  of  ants,  a  smaller,  war-like  species.  These 
ants,  attracted  by  the  much-prized  food,  make  their  home 
on  the  tree  or  in  special  cavities  in  it,  and  repel  all  at- 
tempts of  the  leaf-cutting  species  to  reach  the  foliage. 


Fig.  234.— Epiphytic  group  of  bromeliads  and  orchids  on  a  tree,  in  Cuba 
(Photo  by  M.  T.  Cook.) 

Such    trees   are    called  ant-loving    (myrmecophilous),   or 
myrmecophytes. 

307.  Epiphytism.— Any  plant  (whether  parasite  or  not) 
that  lives  on  another,  or  upon  any  other  convenient 
support  (Fig.  233),  is  an  epiphyte,  but  the  term  is  com- 


328  STRUCTURE    AND    LIFE    HISTORIES 

monly  restricted  to  those  cases  where  there  is  no  phys- 
iological or  nutritional  relationship  between  the  two.  A 
vine  climbing  up  a  tree  is  an  epiphyte,  as  are  also  the 
Pleurococcus  and  mosses  growing  on  the  tree  trunks.  Epi- 
phytism  is  specially  common  in  the  tropics  where  orchids, 


Fig.  235.' — An  epiphytic  Clusia.      (From  photo  by  G.  V.  Nash, 
taken  in  Flaiti.) 

ferns,  hohenbergias,  and  great  lianas  (vines)  are  found 
growing  in  profusion  on  other  plants  (Figs.  234,  235,  and 
236). 

308.  Mutualism. — The    associating    together    of    two 
plants    in    intimate    physiological   relationship,    to    their 


SAPROPHYTISM  AND   SYMBIOSIS 


329 


Fig.  236.- 


-An  orchid  {Cattleya  sp.)  growing  as  an  epiphyte  on  a  portion 
of  a  branch  of  white  birch.     Note  the  aerial  roots. 


Fig.  237. — A  thallose  lichen,  Physcia  stellaris  (L.)  Nyb.,  growing  on  a 
rock.  The  cup-shaped  structures  are  the  fruiting  bodies  (apothecia). 
At  the  left  are  seen  two  very  young  plants,. 


330 


STRUCTURE    AND    LIFE   HISTORIES 


Fig  238.— A  lichen,  Parmelia  perlata  (L.)  Ach.  i.  Plant,  slightly 
reduced  in  size;  a,  apothecia;  b,  lobe  of  thallus;  c,  soredial  patches.  The 
fI\AtZ^7^^^^fT  ^^P^od^f  tive  bodies  composed  of  both  algal  and  fun- 
gal elements,  and  there/ore  able  to  reproduce  the  lichen;  the  ascospores. 


SAPROPHYTISM   AND   SYMBIOSIS  33 1 

mutual  advantage,  is  admirably  illustrated  by  lichens. 
These  plants  grow  commonly  on  the  trunks  and  stems  of 
trees,  on  old  boards  and  fences,  and  on  rocks  (Fig.  237). 
The  plant  body  is  a  thallus,  and,  when  its  inner  structure 
is  examined,  it  is  seen  to  be  a  composite  plant,  formed  by 
a  species  of  green  alga,  resembling  Pleurococcus,  and  sur- 
rounded by  the  mycelium  of  a  filamentous  fungus  (Fig. 
238).  If  supplied  with  suitable  moisture,  the  alga  can 
live  alone,  because  it  has  chlorophyll,  but  the  fungus, 
not  having  chlorophyll,  cannot  live  alone.  By  uniting 
into  a  common  body,  each  plant  supplies  what  the  other 
needs. 

The  fungal  portion  of  lichens  reproduces  by  means  of 
spores  borne  in  asci,  and  is  therefore  an  ascomycete.  The 
apothecium  ("fruiting"  portion  of  the  lichen)  is  in  reality 
a  modified  ascocarp  (Fig.  238).  In  some  species  the 
apothecia  occur  at  the  summit  of  specialized,  upright 
branches  or  podetia  (Fig.  239).  Only  a  few  years  ago 
the  interesting  discovery  was  made  that  lichens  may  be 
experimentally  produced  by  the  artificial  union  of  certain 
algae  and  fungi  (Fig.  240).  Some  of  the  lichens  thus  pro- 
duced resembled  those  found  in  nature,  while  other  com- 
binations were  entirely  new. 

An  interesting  case  of  the  symbiotic  association  of  four 
genera,  if  not  to  their  mutual  benefit,  at  least  without 
apparent  detriment  to  either,  is  found  in  the  roots  of  some 
of  the  Cycadaceae.     All  the  genera  of  this  family  produce 

alone,  cannot  do  this.  2,  Longitudinal  section  of  apothecium;  a,  thecium; 
h  and  c,  the  two  layers  of  the  hypothecium;  d,  upper  algal  layer;  e,  colonies 
of  algae  distributed  through  the  medullary  layer;  /,  lower  algal  layer;  g, 
lower  cortical  layer.  3,  Cross-section  of  vegetative  portion  of  thallus.  4, 
Paraphyses  (sterile  fungal  filaments),  and  spore-sac  (ascus),  containing 
ascospores.  5,  Ascospores.  6,  Algal  cells,  surrounded  by  fungal  hyphae 
with  haustoria  (absorbing  branches).     (After  Schneider.) 


332 


STRUCTURE    AND    LIFE    HISTORIES 


branched,    coralloid   nodules   on   their   roots  (Fig.   241). 
These  roots  are  caused  primarily  by  infection  with  the 


Fig.  239. — 'Thallus  and  erect  portions  (podetia)  of  two  species  of  lichen 
of  the  genus  Cladonia.  The  larger  podetium  is  proliferating,  and  bears  a 
number  of  apothecia  on  the  margin  of  the  podetium  of  the  first  rank. 
Apothecia  are  also  shown  on  the  margin  of  the  larger  podetia  at  the  right. 
(Drawn  from  nature.) 


Fig.  240. — A  lichen  produced  synthetically  by  cultivating  the  spores 
of  a  fungus  in  association  with  a  green  alga.  The  culture  was  carried  on 
in  a  test-tube  on  sterilized  culture  media,  thus  preventing  contamination 
by  bacteria  and  foreign  spores.     (After  Bonnier.) 

nitrogen-fixing    organism,    Pseudomonas    radicicola.     By 
the  development  of  lenticels  at  the  base  of  each  nodule 


SAPROPHYTISM   AND   SYMBIOSIS 


333 


the  epidermis  is  ruptured,  thus  affording  entrance  to 
another  nitrogen-fixer,  Azotobacter,  and  also  under  favor- 
able conditions,  to  a  blue-green  alga  (Nostoc).  This  is  the 
only  known  case  in  which  four  organisms  are  associated 


Fig.  241.— Root-nodules    of    Cycas  revoluta.     n,  one    of    several  cross- 
sectional  views,  showing  the  zone  of  the  symbiont  alga,  Nostoc. 

together  symbiotically.  The  alga  has  never  been  found 
in  the  nodules  of  Bowenia,  Ceratozamia,  Macrozamia,  nor 
Zamia  (all  genera  of  cycads). 

309.  Grafting.— One  of  the  oldest  practices  of  horti- 
culture is  that  of  grafting  a  twig  or  a  bud  of  one  species 


334 


STRUCTURE    AND    LIFE   HISTORIES 


(i)  Root  grafting  in  its  differ- 
stock  prepared  to  receive  the 


Fig.  242. — Various  methods  of  grafting. 
ent  stages,     a,  Scion  cut  for  insertion;  b, 

scion;  c,  stock  and  scion  united;  d,  the  same  tied  up  with  waxed  cord. 
(2)  Cleft  grafting  (Herbaceous),  a,  Scion  ready  for  insertion;  b,  stock; 
c,  stock  and  scion  united;  d,  the  same  tied  up  with  raffia;  e,  cleft  grafting 
(woody).  Stock  with  two  scions.  (3)  Saddle  grafting,  a,  Scion;  b, 
stock;  c,  scion  and  stock  joined.  (4)  Budding,  a,  Budstick;  b,  T-shaped 
cut  in  bark  of  stock;  c,  bud  ready  for  insertion;  d,  stock  with  bud  inserted; 
e,  the  same  tied  up  with  raffia.     (From  Brooklyn  Botanic  Garden  Leaflets.) 


SAPEOPHYTISM  AND   SYMBIOSIS 


335 


onto  the  main  stem  or  branch  of  another  species.  This  is 
accomplished  by  placing  the  freshly  cut  surface  of 
the  scion   against  the   freshly  cut  surface  of  the  stock, 


Fig.  243.— a  tomato   {Ly coper sicmn),  grafted  on  a  potato   {Solatium) 
Note  the  potato  tuber  on  the  surface  of  the  soil.     Cf.  Fig.  404. 

in  such  a  way  that  the  cambium  layers  of  both  come  in 
contact,  and  then  binding  the  two  together  (Fig.  242). 
In    time   the  two  tissues  become   firmly  united,   grow- 


336 


STRUCTURE   AND    LIFE   HISTORIES 


ing  as  one  continuous  stem.  In  all  grafting  the  scion 
maintains  essentially  its  true  nature,  seldom,  if  ever,  being 
affected  by  the  characteristics  of  the  stock,  which  only 
serves  as  a  channel  for  the  passage  of  water  and  food  ele- 
ments to  the  scion,  and  receiving  in  return  from  the  scion 
the  elaborated  carbohydrate  and  other  food  (Figs.  243 
and  404).  Scion  and  stock  therefore  represent  a  case  of 
symbiosis  artificially  brought  about.  In  some  cases 
branches  of  the  same  tree  rub  against  each  other  until 
the  bark  is  worn  through,  bringing  the  cambial  layers  in 
contact,   and  resulting  in   a   '^ natural"   graft. 

310.  Mycorrhizas. — The  roots  of  many  plants  (espe- 
cially of  woody  plants)  enter  into  intimate  association 
with  the  mycelia  of  various  fungi  growing  in  the  soil. 


^.^, 


Fig.  244. — Ectotrophic  micorhizas.  At  left,  micorhizal  mantle  on  root 
of  hickory  {Carya  ovata),  in  cross-section;  at  right,  root- tip  of  an  oak 
{Quercus),  covered  by  fungus  mantle.     (After  W.  B.  McDougall.) 

The  mycelia  either  form  a  mantle  or  jacket  at  or  near 
the  surface  of  the  young  roots  {ectotrophic,  Fig.  244),  or 
they  penetrate  through  the  cell-walls  into  the  cell-cavities 
{endotrophic,  Fig.  245).  Recent  careful  studies  seem  to 
demonstrate  that  the  ectotrophic  mycorrhizas,  common 
on  the  roots  of  many  kinds  of  trees  (hickory,  oaks,  birch, 
sugar-maple,   larch,    beech,    hornbeam),   are,   in   reality. 


SAPROPHYTISM   AND    SYMBIOSIS  337 

evidence  of  the  parasitism  of  some  fungus  on  the  tree. 
Several  species  of  fleshy  fungi  are,  in  this  way,  parasitic 
or  partly  parasitic.  In  ectotrophic  infection  the  hyphae 
penetrate  through  the  epidermis  and  then  grow  and  branch 
underneath  it,  some  of  the  branches  growing  between  the 
individual  epidermal  cells  by  dissolving  the  middle  lamella 
with  enzymes  which  they  secrete.  In  this  manner  there 
is  formed  a  pseudo-tissue,  closely  analogous  to  a  lichen, 
which  replaces  the  true  epidermis.  The  fungus  doubtless 
derives  some  nourishment  from  the  dissolved  (digested) 


Fig.  245.— Tangential  section  of  root  of  the  red  maple  {Acer  rubrum), 
showing  endotrophic  micorhiza  in  the  cells.     (After  W.  B.  McDougall.) 

substance  of  the  middle  lamella,  as  well  as  from  nutrient 
substances  that  diffuse  out  from  adjacent  cells;  but  there 
is  no  evidence  that  the  tree  is  in  any  way  benefited  by  the 
presence  of  the  fungus. 

In  some  cases  the  development  of  the  mycorrhiza- 
mantle  inhibits  the  growth  of  the  root,  and  stimulates  a 
profuse  branching,  which  is  repeated  as  the  branches  are 
infected.  This  gives  rise  to  a  malformation  known  as 
''coral  root,"  which  is  so  well  developed  in  one  herbaceous 
species  as  to  give  the  plant  its  scientific  as  well  as  its 
common  name  — Corallorhiza,  or  coral-root. 

In  endotrophic  mycorrhizas  the  hyphae  penetrate 
through  the  cell-walls  into  the  cell-cavities,  and  in  such 


338 


STRUCTURE    AND    LIFE    HISTORIES 


cases  all  stages  are  found  from  true  parasitism  of  the  fungus 
on  the  root,  through  a  mutually  beneficial  relationship, 
to  a  parasitism  of  the  root-cells  on  the  fungus.  Among 
common  trees  having  endotrophic  mycorrhizas,  may  be 


Fig.  246. — Cancer-root  {Conopholis  americana),  of  the  Broom-rape 
family  {Orohanchacea).  The  ovaries  are  developing  into  capsules.^  The 
plant  derives  its  name  from  the  scaly  cone.  (Photo  by  Elsie  M. 
Kittredge.) 

mentioned  the  black  maple,  horse-chestnut,  and  black 
walnut,  and  most  of  the  heath  family  (Ericaceae),  such 
as  trailing  arbutus,  huckleberry,  wintergreen,  heather, 
laurel,   and  rhododendron.     This  is  probably  the  chief 


SAPROPHYTISM  AND   SYMBIOSIS 


339 


reason  why  it  is  so  difficult  to  transplant  many  of  the 
heaths;  the  delicate  adjustment  between  the  plant  and 
the  mycorrhizal  fungus  is  disturbed  in  transplanting,  and 
the  soil  conditions  in  the  new  habitat  are  not  favorable 
to  its  reestablishment  before  the  plant  dies. 

The  Indian-pipe  and  the  false  ''beech-drops''  (Figs.  230 
and  231),  both  belonging  to  the  heath  family,  also  pos- 
sess endotrophic  mycorrhizas. 


Fig.  247.— Dodder  {Cuscuta  sp.),  parasitic  on  geranium  {Pelargonium). 
A  few  seedlings  at  the  left  are  still  rooted  in  the  soil,  and  are  not  yet  at- 
tached to  the  host-plant.     They  eventually  sever  all  relation  with  the  soil. 

311.  Parasitism. — In  some  cases  of  symbiosis,  as  stated 
above,  only  one  plant  derives  any  benefit  from  the  union, 
which  may  or  may  not  be  of  positive  injury  to  the  other. 
Such  is  the  case  with  the  endotrophic  mycorrhizas,  already 
mentioned.     There  are  many  instances  of  the  parasitism 


340 


STRUCTURE    AND    LIFE    HISTORIES 


of  one  flowering  plant  on  another  (Fig.  246).  In  some  of 
these  cases  as,  for  example,  the  dodder  (Cuscuta),  the  para- 
site may  have  completely  lost  the  power  of  elaborating 
chlorophyll,  and  thus  lack  the  function  of  photosynthesis; 


Fig.  248. — Dodder  {Cuscuta  sp.),  in  flower.     Parasitic  on  a  golden  rod 
{Solidago  ulmifolia).     (Photo  by  Elsie  M.  Kittredge.) 


the  parasitism  is  then  complete  (Figs.  247,  248  and  249). 
In  other  cases  the  parasite  may  retain  its  chlorophyll- 
apparatus,  and  hence  be  only  partly  dependent  upon 
the  host,  as  in  the  case  of  the  mistletoe  (Fig.  250). 
Such    plants    are    semi- parasites.     Another    example    of 


SAPROPHYTISM   AND   SYMBIOSIS 


341 


semi-parasitism  is  that  of  the  blue-green  alga,  Nostoc, 
species  of  which  grow  in  little  pockets  or  cavities  in  the 
tissues  of  the  water-fern  Salvinia,  of  Gunnera  manicata, 


luG.  249.— Photomicrograph  of  a  cross-section  of  the  stem  of  a  dicoty- 
ledonous host-plant  infested  with  the  parasite,  dodder  {Cusciila  sp.). 
Note  the  haustoria  extending  from  the  dodder  (D,  D')  into  the  cortex  of 
the  host  (H).     Greatly  enlarged. 

of   Anthoceros,   and   of   other  plants,    without   apparent 
injury  to  the  host  (Fig.  160). 

312.  Artificial  Parasites. — By  recent  experiments  cer- 
tain plants  have  been  induced,  by  experimental  treatment, 


342 


STRUCTURE   AND    LIFE   HISTORIES 


to  grow  as  parasites  on  other  plants  (Fig.  2500).  The  con- 
dition to  success  in  such  experiments  is  that  the  osmotic 
strength  of  the  cell-sap  of  the  host  must  be  less  than,  or 
at  least  not  greater  than  that  of  the  parasite. 


Fig.  250, — Cross-section  of  a  branch  of  live  oak,  showing  live  stems 
of  mistletoe,  parasitic  on  the  oak;  the  upper  stem  with  foliage  and 
fruit.  Note  the  prominent  "sinkers"  of  the  parasite,  some  of  them 
growing  laterally  for  a  short  distance,  close  under  the  surface  of  the 
bark,  and  then  radially,  deep  into  the  tissue  of  the  wood. 

313.  Fungal  Parasites. — Mention  has  already  been 
made  in  Chapter  XIV  of  the  parasitism  of  the  entire  group 
of  fungi,  including  the  smuts,  rusts,  and  other  disease- 
producing  fungi,  on  flowering  plants.  The  ''shelf-fungi," 
commonly  found  on  forest  trees,  are  economically  impor- 


SAPROPHYTISM   AND    SYMBIOSIS  343 

tant  because  of  the  enormous  financial  losses  occasioned 
by  the  timber-decay  which  they  induce. 

314.  Parasitism  Means  Degeneration. — Most  parasites 
among  the  flowering  plants  have  suffered  the  loss  of  some 
organ  or  organs,  and  of  one  or  more  functions  as  a  result 
of  the  parasitic  habit.  In  fact,  parasitism  must  be  re- 
garded as  an  acquired  habit,  and  the  parasite  among 
plants,  as  in  human  society  or  elsewhere,  as  a  degenerate 
form  of  life.  Some  plants  can  live  only  as  parasites 
{obligate  parasitism),  while  others  may  live  either  as 
parasites  or  as  saprophytes  (facultative  parasitism). 


Fig.  250,  a. — Cissus  laciniata,  parasitic  on  the  cactus  {Opuntia  Blakeana). 
The  parasitism  was  artificially  induced  (xeno-parasitism).  The  host 
plant  has  been  sectioned  to  expose  the  roots  of  the  xeno-parasite.  (Re- 
drawn from  D.  T.  MacDougal.) 

316.  Flowers  and  Insects. — The  dependence  of  certain 
plants  upon  insects  to  secure  the  transfer  of  pollen  from 
one  flower  or  plant  to  another,  will  be  mentioned  more 
in  detail  in  Chapters  XXVII-XXIX. 


CHAPTER  XXII 

THE  PROBLEM  OF  SEX  IN  PLANTS 

316.  Cell-division  and  Reproduction. — As  stated  in 
Chapter  XIV,  the  essence  of  reproduction  is  the  separation, 
from  the  body  of  the  parent,  of  a  cell  or  larger  portion, 
which  becomes  the  starting  point  of  a  new  individual. 
In  some  of  the  lowest  plants,  such  as  certain  species  of 
bacteria,  cell-division  always  results  in  reproduction;  that 
is,  the  two  halves  of  the  divided,  one-celled  body  always 
separate  at  the  close  of  cell-division,  thus  giving  rise  to 
two  new  individuals.  A  little  higher  in  the  scale  of  life 
we  find  such  plants  as  Pleurococcus,  where  cell-division 
may  result  at  once  in  reproduction,  but  where  there  is 
also  a  marked  tendency  for  the  cells  to  adhere  together 
at  the  close  of  division,  thus  forming  a  loosely  organized, 
multi-cellular  plant  body  (Fig.  183) .  A  further  advance  is 
illustrated  by  Spirogyra,  where  the  cells  normally  do  not 
separate  at  the  close  of  division,  but  remain  together,  end 
to  end,  producing  a  multi-cellular  body  in  the  form  of  a 
filament  (Fig.  251).  From  this  simple  condition  we  have 
seen  transitions  to  the  flat  thallus  of  the  liverworts;  the 
simple,  leafy  axis  of  the  mosses,  the  leafless  axis  of  the 
moss-sporophyte,  and  the  leafy  sporophyte  of  the  ferns. 
Not  that  these  forms  are  derived  from  each  other;  tut  they 
illustrate  various  degrees  of  complexity  from  the  simplest 
unicellular  plant  body  to  a  complex,  multi-cellular  body. 
By  a  comparison  of  these  forms,  we  see  that  while,  in 

344 


THE  PROBLEM  OF  SEX  IN  PLANTS 


345 


some  of  the  lowest  forms,  cell-division  always  results  in 
reproduction,  in  the  higher  forms  it  only  paves  the  way 


Fig.  251.-  Spirogyra  sp.  A,  terminal  portion  of  vegetative  filament; 
B,  stages  of  scalariform  conjugation;  C,  preparation  for  lateral  conjuga- 
tion; Z>,  zygospores  formed  by  lateral  conjugation. 

to  increasing  the  size  of  the  given  individual,  that  is,  to 
growth.     In  other  words,  between  cell-division   and  re- 


346  STRUCTURE    AND    LIFE   HISTORIES 

production  there  is  interposed  the  enlargement  or  growth 
of  the  parent. 

317.  Vegetative  Multiplication. — Attention  has  also 
been  called  to  the  various  methods  by  which  the  number 
of  individual  plants  is  increased  by  the  separation  of  multi- 


FiG.  252. — Branch  of  a  willow  (Salix  sp.),  showing  the  formation  of 
fibrous  roots.  The  lower  portion  of  the  stem  was  placed  in  water  for  a 
few  days. 

cellular  portions  of  the  body  of  the  parent.  The  most 
familiar  of  these  processes  being  the  artificial  propagation 
of  plants  by  means  of  cuttings.  A  portion  of  stem  (Figs. 
3  and  252),  or  sometimes  of  leaf  (Fig.  253),  stuck  into 
moist  sand  will  form  new  roots  and  ultimately  develop  an 


THE  PROBLEM  OF  SEX  IN  PLANTS 


347 


entire  new  plant.  A  leaf  of  begonia  laid  on  moist  sand 
will  give  rise  to  several  new  plants  wherever  it  is  cut  (Fig. 
254).     Leaves  of  the  sundew  (Drosera)  frequently  strike 


Fig.  253. — Regeneration  at  the  leaf-base  of  potato  leaves  {Solanum 
tuberosum),  a,  roots  formed;  h,  tuber-like  enlargement;  c,  same  as  h, 
with  roots;  d,  formation  of  true  tuber.     (After  Miss  Elsie  Kupfer.) 


Fig.  254. — Young  plantlets  developing  from   the  edges  of   lacerations 
made  in  a  large  leaf  of  Rex  begonia. ' 

root  at  the  tip  and  develop  new  plants  (Fig.  255),  while 
the  leaves  of  Bryophyllmn  normally  produce  marginal 
buds  from   which   new   plants   develop   (Fig.    256).     As 


34S 


STRUCTURE    AND    LIFE    HISTORIES 


noted  in  Chapter  XVI,  the  thallus  of  a  liverwort  may  be 
chopped  fine  and  every  isolated,  intact  cell  will  give  rise 
to  a  new  plant. 

Growing  plants  of  the  liverwort,  Marchantia,  isolated 
by  the  dying  of  older  tissue  develop  new  individuals; 
the  tips  of  the  leaves  of  the  walking  fern  may  strike  root 


Fig.  255. — Drosera  rotmidifolia.     Production  of  a  young  plant  from  the 
leaf  of  an  older  plant. 

and  originate  new  plants  (Fig.  122),  the  tips  of  stolons  or 
runners  (as  in  ferns,  eel-grass,  strawberries,  etc.)  may 
do  the  same  (Figs.  257-259,  and  123),  isolated  sterile 
branches  and  "innovation-branches"  of  Sphagnum  moss 
become  new  individuals  (Fig.  144),  as  may  also  the  famihar 
'.ubers  and  bulbs  (such  as  those  of  the  potato  and  onion), 


THE  PROBLEM  OF  SEX  IN  PLANTS 


349 


Fig.  256. — Bryophyllum  crenatum.     A  leaf  which  has  given  rise  to  three 
plantlets  along  the  margin  of  the  blade. 


Fig.  257. — Eel-grass  {Valtisneria  spiralis),  showing  vegetative  propaga- 
tion by  stolons.     Young  plants  at  P^  and  P^. 


350 


STRUCTURE    AND    LIFE    HISTORIES 


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Fig.  258. — Scmperviviim  tectoruniy  with  stolons. 


Fig.  259. — Chlorophytum  elatmn  R.  Br.,  showing  vegetative  propagation 
by  runners.  P^,  P^,  P^,  new  plants;  P,  parent  plant.  From  P^  one  root, 
and  from  P^  two  roots  hang  down. 


THE  PROBLEM  OF  SEX  IN  PLANTS 


351 


and  the  gemmcB  of  various  species  of  liverworts,  mosses, 
and  ferns  (Fig.  260). 

An  interesting  form  of  vegetative  multiplication  is 
illustrated  by  the  fern  Woodwardia  orientalis,  where 
new  plantlets  arise  at  numerous  points  on  the  upper 
surface  of  the  leaves  (Fig.  261).  The  number  of  in- 
dividuals is  thereby  increased  or  multiplied,  hence  the 
term  vegetative  multiplication. 


Fig.  260.— Hymenophyllum    sp.     Prothallus.     a,    antheridia;    b,    arche- 
gonia;  g,  gemmae.     (After  Winifred  J.  Robinson.) 

318.  Reproduction  by  Spores.— In  many  plants  such, 
for  example,  as  the  fern,  the  parent  plant,  while  retaining 
its  own  vegetative  organs  intact,  gives  off  individual 
cells  (the  spores),  which  become  the  starting  points  of 
new  individuals.  The  most  distinctive  thing  about  a  spore 
is  that  it  escapes,  or  becomes  separated  from  the  parent,  while 
all  the  other  cells  remain  organically  united. 

In  some  one-celled  plants  {e.g.,  yeast.  Fig.  67),  the  en- 
tire plant  body  (except  the  cell-wall)  may  become  organ- 


352  STRUCTURE    AND    LIFE    HISTORIES 

ized  into  spores,  which  escape  from  the  cell-wall  of  the 
mother-cell.  In  a  number  of  many-celled  plants  {e.g., 
Ulothrix)  practically  every  protoplast  has  the  capacity  of 
becoming  organized  into  one  or  more  spores  which  escape 
from  the  old  cell-cavity.  The  next  higher  step  is  the 
restriction  of  spore  formation  to  certain  cells  in  special 
organs  (sporangia),  while  the  other  cells  function  only 
vegetatively. 


'iG.  261. —  IVoodwardia  orientalis.     Portion  of  a  leaf  bearing  numerous 
young  plantlets  on  its  upper  surface. 


319.  Cell-fusions. — Through  all  the  variations  of  re- 
production by  spores  there  is,  as  a  rule,  only  the  separation 
of  protoplasts  from  the  parent  body,  never  a  cell-fusion  or 
nuclear-fusion.  Some  plants,  however,  such  as  Ulothrix 
(Fig.  262),  have  been  found  to  produce  two  sizes  of 
spores,  and  the  small  spores  must  always  unite  before 
they  can  develop  into  full-sized,  mature  individuals.^ 
Attention  has  already  been  called  (Chapter  XVIII)  to 
the  condition  of  similar  sized  gametes  {isogamy),  as  in 
Spirogyra,  in  contrast  to  that  of  unequal  gametes  (hetero- 
gamy), as  in  Ascophyllum  and  Fuciis. 

1  In  certain  cases  {e.g.,  Ulothrix)  the  microspores  may  develop  small, 
imperfect  individuals  without  fusion. 


THE  PROBLEM  OF  SEX  IN  PLANTS  .       353 

320.  Maleness  and  Femaleness. — Spirogyra.  In  nearly 
all  cases  of  cell-fusion  it  is  possible  to  recognize  some 
difference,  either  of  structure  or  of  behavior,  between  the 
gametes.  In  Spirogyra,  for  example,  it  has  long  been 
noticed  that  if  one  of  the  cells  of  a  filament  passes  over  to 
the  other  filament  through  the  conjugation-tube,  all  the 
cells  of  that  filament  will  ordinarily  do  the  same.  Thus 
after  conjugation  is  over  all  the  cells  in  one  filament  will 
be  found  empty,  while  all  the  cells  of  the  adjacent,  con- 
jugating filament  will  contain  zygotes.  This  behavior, 
however,  varies  under  certain  conditions  and  with  differ- 
ent species. 

Recent  studies  by  York  have  revealed  the  fact  that  the 
supplying  gamete  of  Spirogyra  always  possesses  less  starch 
and  a  less  number  of  starch-formers  (pyrenoids)  than  does 
the  receiving  gamete  (Fig.  263).  It  has  also  been  noted 
that  the  supplying  filaments  (male  ?)  are  less  vigorous, 
vegetatively,  than  the  receiving  filaments  (female  ?). 

321.  Sexuality  in  Molds.— One  of  the  most  interesting 
of  recent  discoveries  in  connection  with  sex  in  plants,  is 
that  of  the  existence  of  two  strains  of  different  sexual  value 
in  the  molds.  It  was  known  for  a  long  time  that  conjuga- 
tion and  the  formation  of  zygotes  in  these  plants  could  not 
always  be  secured  when  desired;  that  is,  conjugation  would 
not  occur  between  every  two  individuals.  At  first  it  was 
thought  that  the  explanation  lay  in  the  fact  that  the  ex- 
ternal conditions  (temperature,  light,  moisture),  might 
not  be  just  right,  or  that  the  two  adjacent  plants  were 
not  in  the  right  condition  as  to  age,  or  nutrition,  or 
otherwise.  Finally,  as  stated  in  Chapter  XIX,  Blakeslee, 
by  careful  experimental  studies,  found  that  there  are  two 
unlike  strains  of  many  of  the  molds,  and  that  whenever  a 

23 


354 


STRUCTURE    AND    LIFE    HISTORIES 


to  o 
o  O- 


u  c  o    .s  o 
— '  en   (/I  ^>   N 


THE  PROBLEM  OF  SEX  IN  PLANTS 


355 


plant  of  one  strain  grows  next  to  a  plant  of  another 
strain,  conjugation  will  always  take  place  between  them. 
These  strains  he  designated,  provisionally  as  (+)  and  (— ) 
(Fig.  190).     The  f+)  strain  is  vegetatively  more  vigorous 


Fig.  263, — Spirogyra  sp.,  illustrating  sexual  differentiation.  ^  Receiving 
(female)  gamete  at  the  left;  supplying  (male)  gamete  at  the  right.  (Re- 
drawn from  camera  lucida  drawing  by  H.  H.  York.) 


than  the  (— )  strain,  and  the  conclusion  seems  warranted 
that  the  (+)  race  is  female  and  the  (— )  race  male. 

322.  Sexual  Differentiation  of  Spores. — i.  Physiolog- 
ical. Even  an  elementary  study  of  reproduction  reveals 
the  fact  that  spores  from  the  same  plant,  and  even  from 
the  same  sporangium  (as  in  some  of  the  molds  just  men- 


356 


STRUCTURE    AND    LIFE    HISTORIES 


tioned),  and  to  all  external  appearances  entirely  alike, 
may  produce  individuals  of  different  sex- value,  some  being 
male  and  some  female.-^  Such  was  also  seen  to  be  the 
case  with  the  marine  alga  Dictyota,^  the  externally  similar 
male  and  female  gametophytes  being  produced  by  spores 
that  are  alike  in  size  and  other  external  features. 

As  a  further  advance  spores  that  appear  to  be  morpho- 
logically alike  may  produce  plants  morphologically  as  well 
as  physiologically  different  {Anthoceros,  some  molds). 

2.  Structural.  In  the  little  club-mosses  {Selaginella) 
we  found  the  spores  unlike,  not  only  in  function  but  in 
structure,  those  producing  males  being  smaller  than  those 
producing  females — the  condition  of  heterospory. 


Fig.  264. — Vaucheria  terrestris.    anth,  antheridium  (empty);  o,  oogonia. 


323.  Differentiation  of  Sex-organs. — In  many  lower 
plants  there  is  no  recognizable  structural  difference  be- 
tween the  organs  that  produce  the  heterogametes,  but  a 
step  in  advance  in  this  direction  is  found  in  such  a  plant 
as  Vaucheria  (Fig.  264),  where  the  antheridia  are  struc- 

1  See,  also,  pp.  246  and  248. 

2  See  Chapter  XVIII. 


THE  PROBLEM  OF  SEX  IN  PLANTS         357 

turally  very  unlike  the  oogonia.  This  differentiation  is 
carried  a  step  further  with  the  appearance  of  the  multi- 
cellular archegonium  in  liverworts  and  mosses. 

324.  Structural  Differentiation  of  the  Sexes. — i.  Par- 
tial.— In  some  species  (though  not  in  all)  of  the  fresh- 
water alga,  CEdogonium,  the  spores  are  unlike.  Large 
zoospores  produce  normal-sized  plants  that  bear  eggs 
and  smaller  androspores.  The  androspores,  intermediate 
in  size  between  sperms  and  zoospores,  produce  smaller, 
male  indimdiials  only,  of  simple  structure,  which  fasten 
themselves  to  the  egg-bearing  plants,  and  give  rise  to 
sperms,  which  fertilize  the  tgg.^ 

2.  Complete. — In  the  liverworts  and  mosses  commonly, 
and  in  the  higher  plants  always,  the  gametophytes  are 
clearly  differentiated  into  male  and  female,  with  unlike 
vegetative  characters  which  clearly  distinguish  them. 
These  unlike  structural  features  are  called  the  secondary 
sexual  characters.  In  these  groups  the  sporophytes  are 
not  differentiated  in  structure,  but  the  spores  they  pro- 
duce, though  structurally  alike,  are  physiologically  unlike, 
some  producing  male  gametophytes,  others  female. 

The  most  complete  expression  of  maleness  and  female- 
ness  is  found 'in  certain  seed-bearing  plants,  where  the 
sporophytes  are  differentiated  -  into  microsporophytes 
(staminate  plants),  bearing  only  microspores  which  pro- 
duce male  gametophytes  (pollen-grains),  and  megasporo- 
phytes  (pistillate  plants),  bearing  only  megaspores  which 
produce  female  gametophytes  (embryo-sacs) .  Usually  the 
two  kinds  of  sporophytes  are  essentially  alike,  except  for 
the  sporophyll-bearing  branches   (the  flowers),   but  the 

^  It  has  been  suggested  that  these  androspores  might  be  regarded  as 
sperms  developing  without  fertilization,  i.e.,  by  parthenogenesis. 


358  STRUCTURE    AND    LIFE    HISTORIES 

male  and  female  gametophytes  are  as  unlike  as  could  well 
be  imagined.  Illustrations  of  this  condition  will  be 
found  when  we  study  the  Gymnosperms  (zamia,  cycas, 
ginkgo),  and  the  Angiosperms  (willow,  poplar,  hop,  etc.). 

325.  Determination  of  Sex. — i.  Effect  of  Nutrition. — 
Nobody  knows  the  real  cause  of  sex — of  maleness  and 
femaleness.  We  may  arrange  plants  (and  animals)  in 
a  series  so  as  to  show  the  gradual  transition  from  the 
simple  non-sexual  condition  to  complete  differentiation  of 
males  and  females,  but  in  doing  this  we  should  clearly 
recognize  the  fact  that  we  have  really  explained  nothing. 
We  have  only  described  events  and  structures  in  the  order 
in  which  it  seems  probable  that  they  have  occurred,  in 
the  gradual  development  of  the  earth's  vegetation. 

But  while  we  have  never  yet  been  able  to  determine  in 
advance  the  sex  of  a  plant  or  animal,  we  have  been  able 
to  determine  which  sex  shall  gain  expression.  For  ex- 
ample, we  have  seen  above  that  male  plants  are  frequently 
less  vigorous  and  more  poorly  nourished  than  female 
plants.  We  would,  therefore,  expect  that  poor  nutrition 
would  cause  a  suppression  of  femaleness,  and  this  is  pre- 
cisely what  has  been  found  in  certain  experiments  that 
have  been  made.  When  the  prothallia  of  certain  ferns 
that  normally  bear  both  antheridia  and  archegonia  are 
grown  under  conditions  that  result  in  their  being  poorly 
nourished,  the  antheridia  develop,  but  not  always  the 
archegonia.  In  such  cases  we  know  that  we  have  not 
changed  the  sexual  nature  of  the  prothallus,  but  have 
only  modified  its  expression.^     This  is  further  illustrated 

^  In  exceptional  cases  perfect  flowers  may  appear  on  staminate  or  pistil- 
ate  plants,  as  in  papaw  (Carica  Papaya)  and  willow,  suggesting  that 
dioecious  plants  may,  in  reality,  be  of  double  sex-value,  but  that  only  one 
of  the  sexes  comes  normally  to  expression. 


THE  PROBLEM  OF  SEX  IN  PLANTS 


359 


in  the  gametophytes  of  the  Horsetails  (Chapter  XXIV), 
which  are  usually  differentiated  into  larger  ones  (female) 
and  smaller  ones  (male);  but  under  certain  conditions 
(apparently  involving  differences  of  nutrition),  the  female 
gametophytes  may  bear  antheridia,  and  the  male  gameto- 
phytes archegonia. 

2.  Efect  of  Constitution  of  Germ-cells. — If  sex  is  not  de- 
termined by  external  conditions — by  environment — then 

Zygote 
Mal9 

(zo  ox  my 


Sperm 
No-X,or  y 

Sperm 
Xr class 


(2Z) 
Zygote 
Female 
Fig.  265, — Diagram  to  illustrate  determination  of  sex  by  the 
ic-chromosome. 

its  explanation  must  lie  in  the  internal  constitution  of  the 
germ-cells — in  their  chemical,  physical,  or  morphological 
differences.  Remarkably  careful  and  accurate  observa- 
tion has,  in  fact,  revealed  a  constant  and  fundamental 
morphological  difference  in  the  germ-cells  of  animals. 
It  has  been  found,  for  example  in  some  insects,  that  the 
nucleus  of  every  egg  possesses  a  certain  clearly  distinguished 
chromosome,    called    the   rr-chromosome,^    while   in    the 

'  The  X,  as  in  algebra,  indicating  an   unknown,  or  not  understood, 
factor. 


360  STRUCTURE    AND    LITE    HISTORIES 

sperms  one-half  possess  it  (x)  and  one  half  do  not  (no  —  x). 
In  other  cases  (first  recorded  for  hemipterous  insects  by- 
Wilson)  the  x-chromosome  in  the  cells  of  the  male  is 
accompanied  by  a  companion  chromosome  of  a  different 
type,  called  by  Wilson  the  y-chromosome.  In  the  reduction 
division  one  half  the  sperms  receive  the  a:-chromosomes, 
the  other  half  the  ^'-chromosomes.  In  some  cases  the 
X-  and  j-elements  are  not  single  chromosomes  but  groups 
of  small  chromatin  bodies.  The  x-element  is  always 
associated  with  femaleness,  the  no  —  x  (or  y)  with  maleness. 
If  an  egg  (x)  is  fertilized  by  a  sperm  possessing  the  x- 
chromosome  a  female  zygote  is  determined  (formula  xx) ; 
the  union  of  an  egg  with  a  no  —  x  or  3;  sperm  results  in 
a  male  zygote  (formula  xo  or  xy) ,  thus : 

Egg  X  -f-  sperm  no  —  x  =  zygote  x  (male). 

Egg  X  +  sperm  y  =  zygote  xy  (male) . 

Egg  X  +  sperm  x  =  zygote  xx  (female) . 
This  condition  is  illustrated  diagrammatically  in  Fig.  265. 
In  sea-urchins  and  some  other  animals  the  condition  may 
be  reversed,  the  sperms  being  all  alike  and  the  eggs  unlike. 
Careful  investigations  have,  so  far,  failed  to  reveal  any- 
thing corresponding  to  the  x-chromosome  in  plants, 
except  in  two  species  of  the  dioecious  liverwort,  Sphcero- 
carpos.  In  1919  Allen^  (for  S.  Donnellii)  and  Miss  Schacke^ 
(for  S.  texanus)  demonstrated  the  presence  in  the  cell- 
nuclei  of  the  female  plants  (gametophytes)  of  one  x- 
chromosome,  clearly  distinguished  from  the  seven  other 
chromosomes  by  its  much  greater  size.     Likewise  they 

1  Allen,  C.  E.     The  basis  of  sex  inheritance  in  Sphcerocarpos.     Proc. 
Amer.  Phil.  Soc,  58:  289-316.     1919. 

2  Schacke,  Martha  A.     A  chromosome  difference  between  the  sexes  of 
Splmrocarpos  texanus.     Science,  N.  S.,  49:  218-219.     Feb.  28,  1919. 


THE  PROBLEM  OF  SEX  IN  PLANTS        36 1 

have  identified  in  the  cell-nuclei  of  the  male  gametophytes 
one  ^^-element,  distinguished  by  its  much  smaller  size 
(Fig.  265,  a).  As  would  be  anticipated,  the  cell-nuclei  of 
the  sphorophyte  generation  contain  both  the  x-  and  the 
^/-elements.  On  the  basis  of  these  discoveries  Prof.  Allen 
predicts  that  further  investigation  is  likely  to  reveal  the 
presence  of  similar  bodies  or  elements  in  other  plants. 

326.  The  Meaning  of  Sex. — Just  what  is  accomplished 
for  plants  by  the  occurrence  of  two  sexes  is  not  entirely 
understood.  Among  the  lower  plants  the  primitive  expres- 
sions of  sex  seem  in  some  cases,  to  have  met  a  need  for  better 
nutrition,  or  to  have  resulted  in  rejuvenating  the  protoplasts 
of  the  gametes;  but  these  explanations  are  not  satisfactory, 
especially  for  the  higher  plants.  We  do  know  that  fertili- 
zation always  results  in  increasing  variation.  When 
plants  are  propagated  vegetatively,  as  by  cuttings  or  by 
grafting,  the  characters  remain  constant  in  the  new 
plants,^  but  when  reproduction  is  by  seeds  (resulting  from 
fertilization)  we  always  observe  great  variation.  This 
is,  of  course,  an  advantage,  for  it  is  only  by  variation  that 
new  characters  may  appear,  and  without  the  appearance 
of  new  characters  there  would  be  no  opportunity  for  the 
improvement  of  plants — either  by  nature  or  by  man. 

The  results  of  all  studies  and  discussions  of  the  question 
of  sex  lead  us  to  recognize  the  fact  that  it  is  still  largely  an 
unsolved  mystery.  We  must  make  further  and  more 
accurate  observations,  and  be  careful  and  logical  in  our 
reasoning,  before  we  can  hope  to  solve  this  difficult 
problem. 

^  Except  in  the  special  case  of  bud-sporting  (p.  532,  and  Fig.  400). 


CHAPTER  XXIII 
FROM  ALGA  TO  FERN 

327.  Progressive  Development. — The  largest  fact  that 
stands  out  in  a  hasty  review  of  the  plants  we  have  studied 
is  the  increasing  simplicity  of  form  and  organization  from 
fern  to  alga,  or  in  reverse  order,  the  increasing  com- 
plexity from  alga  to  fern.  The  Pleurococcus  is  a  simple 
globule  of  living  matter.  Its  organs  are  all  reduced  to 
their  lowest  terms — cell-wall,  cytoplasm,  limiting  surface 
(or  membrane),  nucleus,  nuclear  membrane,  chromato- 
phore — the  parts  of  a  cell.  The  one  protoplast  performs 
all  the  functions  of  life — takes  in  raw  materials,  elaborates 
food  out  of  these  raw  materials,  digests  the  food  thus 
made,  assimilates  it,  respires,  and  reproduces  itself.  It  is 
difficult  to  imagine  the  fundamental  life-functions  per- 
formed under  simpler  circumstances  of  structure. 

But  as  soon  as  plant  cells  begin  to  remain  united  after 
cell-divison  they  begin  to  be  differentiated.  The  single 
Pleurococcus  cell  is  globular,  but  when  two  or  more 
remain  attached  they  are  flattened  at  the  surfaces  in 
contact  (Fig.  183).  This  is  a  simple  illustration  of 
morphological  differentiation.  When  a  cell-mass  is 
formed  the  outer  cell-walls,  in  contact  with  the  air, 
become  covered  with  a  layer  of  cuticle,  which  retards 
the  loss  of  water.  Cell-walls  in  contact  with  each  other 
do  not  possess  the  cuticle.  Thus,  by  gradual  steps  the 
plant  body  becomes  increasingly  complex,  so  that  we 

362 


FROM   ALGA    TO   FERN  363 

may  arrange  a  series  from  algae  to  ferns,  and  from  ferns 
to  the  higher  seed-bearing  plants,  showing  increasing 
complexity  of  structure. 

328.  Division  of  Physiological  Labor. — The  differentia- 
tion of  the  plant  body  into  organs — root,  stem,  leaf,  re- 
productive organs — may  be  considered  as  an  expression  of 
the  division  of  physiological  labor.  For  example,  when  a 
sufficiently  thick  cell-mass  is  formed  the  inner  cells  may 
be  deprived  of  light;  no  chlorophyll  can  then  develop, 
photosynthesis  becomes  impossible,  and  the  outer^cells  must 
elaborate  all  the  food,  not  only  for  themselves,  but  for  the 
inner,  non-green  cells  as  well.  Roots  that  serve  to  hold  the 
plant  in  the  soil,  and  to  take  in  water  and  minerals  to  be  used 
in  the  leaves,  must  be  nourished  by  food  elaborated  in  the 
green  cells.  The  leaves  and  branches  must  be  supplied 
with  water  taken  in  by  the  roots,  and  a  vascular  system 
becomes  necessary.  So,  in  these  and  countless  other  ways, 
the  vegetative  functions  become  divided  among  organs 
specially  fitted  by  their  structure  to  perform  them  well. 
Then  the  reproductive  function  becomes  confined  to 
certain  cells,  which  are  nourished  by  the  others.  Repro- 
duction itself  becomes  complicated  by  the  development 
of  two  kinds  of  gametes,  and  the  introduction  of  cell- 
fusion  as  well  as  cell-division. 

Among  plants,  organization — the  development  of  defi- 
nite organs  for  definite  work — has  the  same  kind  of  ad- 
vantage as  the  division  of  labor  among  men.  The 
"jack-at-all- trades"  is  not  as  efficient  at  any  one  of  them 
as  the  specialist.  The  existence  of  carpenters,  plumbers, 
masons,  tailors,  architects,  superintendents,  teachers, 
lawyers,  stenographers,  doctors,  means  greater  efficiency, 
than  could  be  secured  if  everyone  tried  to  be  all  of  these. 


364  STRUCTURE    AND    LIFE    HISTORIES 

So  the  differentiation  of  the  plant  body  into  root,  stem, 
leaves,  and  organs  of  reproduction  means  greater  efficiency 
in  the  performance  of  all  the  functions  of  life. 

329.  From  Water  to  Land. — A  careful  consideration  of 
all  available  evidence  leads  to  the  conviction  that  plant 
life  originated  in  the  water.  For  example,  the  more 
primitive  types  of  plants  have  no  well-defined  polarity) 
that  is,  they  do  not  present  an  axis  with  the  opposite  ends 
clearly  differentiated  for  the  performance  of  different 
functions  under  unlike  surroundings,  such  as  roots  ad- 
justed to  a  life  in  the  soil  and  darkness,  and  leaves 
adjusted  to  a  life  in  the  air  and  light.  Plants  submerged 
in  water,  such  as  Spirogyra,  commonly  possess  a  uni- 
formity of  structure,  in  harmony  with  their  uniform 
environment.  Of  course,  there  are  exceptions  to  this, 
Ascophyllum,  Dictyota,  Vaticheria,  and  other  submerged 
aquatics  possess,  on  one  end,  hold-fasts  which  anchor  them 
to  the  substratum;  but  these  plants  probably  represent 
early  steps  toward  a  rooted  existence  on  land. 

One  of  the  most  marked  evidences  of  aquatic  life  for 
primitive  organisms  is  their  method  of  reproduction  by 
motile  spores  and  motile  gametes;  while,  at  the  same  time, 
one  of  the  most  distinctive  characteristics  of  the  more 
highly  developed  land-plants  is  reproduction  by  non- 
motile  spores,  suited  to  distribution  by  wind.  The 
enormous  number  of  spores  produced  insures  a  rapid 
multiplication  of  individuals,  and  their  dryness  insures 
protection  during  periods  of  more  or  less  prolonged 
drought. 

330.  Development  of  the  Sporoph3rte. — For  the  suc- 
cessful production  of  large  numbers  of  spores  there  is 
needed  some  provision  for  richly  nourishing  the  spore- 


FROM  ALGA   TO   FERN  365 

producing  tissues.  There  must  be  a  large  amount  of 
chlorophyll-bearing  tissue,  ample  provision  for  taking  in 
abundant  water  and  minerals,  and  efficient  channels  for 
conducting  the  raw  materials  and  elaborated  food  from 
one  part  of  the  plant  to  another.  Moreover,  the  spore- 
bearing  parts  need  to  be  lifted  into  the  air  to  insure  the 
most  efficient  distribution  of  the  spores.  These  needs 
are  admirably  met  by  the  sporophyte  with  roots  on  one 
end,  green  leaves  on  the  other,  and  sporangia  borne  at 
or  very  near  the  tips  of  the  branches. 

A  review,  at  this  time,  of  the  sporophytic  phases  of  the 
liverworts,  mosses,  and  ferns  will  show  how  these  sporo- 
phytes  gradually  increase  in  complexity  and  importance, 
from  the  simple  condition  in  Riccia,  with  almost  no  sterile 
tissue,  through  the  sporogonium  of  the  higher  liverworts 
and  mosses,  to  the  leafy  sporophyte  of  the  ferns.  The 
final  step  in  the  development  of  the  sporophyte  was  the 
differentiation  of  megasporophytes,  bearing  only  mega- 
spores,  and  microsporophytes,  bearing  only  microspores. 

331.  Decline  of  the  Gametophyte. — As  the  sporophyte 
became  more  highly  developed  and  the  dry-land  fio-a 
more  firmly  established,  the  gametophytic  phase  became 
less  essential  and  less  in  evidence,  until,  in  the  ferns, 
the  sporophyte  became  the  commonly  recognized  "plant," 
and  the  very  existence  of  the  gametophytic  phase  was,  for 
a  long  time,  not  known.  Reproduction  by  spores  and 
by  other  non-sexual  means  became  entirely  sufficient  to 
perpetuate  the  race. 

332.  Classification. — By  a  careful  comparison  of  all 
kinds  of  plants,  it  has  been  recognized  that  certain  ones 
are  very  much  alike  in  fundamental  characteristics  of 
structure;  they  fall  naturally  into  a  group.     Moreover  it 


366  STRUCTURE    AND    LIFE    HISTORIES 

is  recognized  that  there  are  numerous  groups,  and  that 
the  members  of  any  one  group  differ  from  those  in  every 
other  group  in  some  fundamental  point.  By  such  com- 
parisons botanists  have  been  able  to  classify  all  known 
plants  into  more  or  less  clearly  defined  groups  and  sub- 
groups. The  larger  the  number  of  characters  considered, 
the  smaller  the  group,  and  vice  versa. 

The  divisions  of  the  plant  kingdom  already  studied,  and 
their  distinguishing  characters  are  as  follows :  ^ 

Divisions  of  the  Plant  Kingdom 

1.  T hallo phytes. — Plant  body  a  thallus;  no  archegonia. 

2.  Bryophytes. — Archegonia;  no  vascular  system. 

3.  Pterido phytes. — Vascular  system;  no  seeds. 

These  four  divisions  are,  of  course,  distinguished  by 
other  characters  than  the  ones  just  indicated,  but  these 
stand  out  prominently  as  positive  and  negative  character- 
istics of  the  respective  groups. 

These  three  divisions  are  further  subdivided,  as  shown 
on  the  following  page. 

'  Adapted  from  Coulter  (J.  M.). 


FROM   ALGA   TO    FERN  367 

Table  IV.— SuBorvrsiONs  of  the  Plant  Kingdom 
I.  Thallophyta 

(a)  Algae 

(i)  Cyanophyceae 

(2)  Chlorophyceae 

(3)  Phaeophyceae 

(4)  Rhodophyceae 

(b)  Fungi 

(i)  Myxomycetes 

(2)  Schizomycetes  (bacteria) 

is)  Phycomycetes 

(4)  Ascomycetes 

(5)  Basidiomycetes 

(6)  Fungi  imperfecti  (life  histories  imperfectly  known). 

(c)  Lichens 

2.  Bryophyta 

(a)  Hepaticae 

(b)  Musci 

3.  Pteridophyta  (true  ferns)  ^ 

(a)  Eusporangiatas 

(b)  Leptosporangiatae 

4.  Calamophyta  (calamites) 
(a)  Equisetineae 

5-  Lepidophyta  (lycopods) 

(a)  Lycopodineae  (homosporus  ciuu-mosses) 
(6)  Lepidodendrineae  (heterosporus  club-mosses) 

*  An  older  classification  combined  the  last  three  groups  or  phyla  into 
one,  as  follows: 
3.  Pteridophyta  (ferns  and  fern  allies) 

(a)   Filicineae 

(6)  Equisetineae 

(c)  Lycopodineae 


CHAPTER  XXIV 
CALAMITES  AND  LYCOPODS 

I.  THE  HORSETAILS  (EQUISETALES) 
EQUISETUM 

333.  Habitat  and  Distribution. — As  the  names  of  some 
of  the  various  species  indicate,  representatives  of  the 
genus  Equisetum  occur  in  a  rather  large  variety  of  habitats. 
Thus  we  have  the  swamp-equisetum  {E.  palustre) ,  meadow- 
equisetum  {E.  pratense),  the  field-equisetum  {E.  arvense), 
and  so  on.  They  are  frequently  found  along  railroad 
embankments  in  exposed  situations,  while  other  species 
occur  only  in  shaded  or  very  moist  locations  (Fig.  266). 
They  are  distributed  throughout  the  northern  hemi- 
sphere, but  only  one  species  has  been  reported  from  South 
America.  Twenty  species  have  been  described  from  the 
temperate  and  tropical  North  America,  but  none  has  ever 
been  found  in  Australia.  Fossils  of  near  relatives  of  the 
genus  have  been  found  in  the  rocks  of  previous  geological 
ages,  and  some  of  the  fossils  in  the  coal-bearing  rocks  of 
the  Carboniferous  age  are  thought  to  belong  to  Equisetum 
itself.  In  temperate  America  the  horsetails  vary  in  height 
from  only  a  few  inches  to  several  feet.  One  species  {E. 
dehile),  found  near  Lahore,  in  India,  attains  a  height  of  from 
10  to  15  feet,  needing  the  support  of  neighboring  trees  in 
order  to  stand  erect,  while  E.  giganteum,  found  from  the 
West  Indies   to   Chili,   reaches    a  maximum    height  of 

368 


CALAMITES    AND    LYCOPODS 


369 


about  40  feet.     In  the  island  of  Jamaica  it  is  found  in 
thickets  10  to  15  feet  high. 

334.  Description  of  the  Sporophyte. — The  plant  body 
(Fig.  267)  consists  of  a  horizontal,  much-branched,  under- 
ground stem,  or  rhizome,  from  which  spring  two  kinds  of 
sub-aerial   branches — sterile  and  fertile.     The  cells  that 


Fig.  266. — Equisetwn  fluviatile.     Pure  stand  in  shallow  water,  at  Tully 
Lake,  N.  Y.     (Photo  by  W.  L.  Bray.) 

compose  the  sub-aerial  branches  are,  in  many  of  the 
species  {e.g.,  E.  aroense),  nearly  or  quite  all  formed  by  the 
close  of  the  growing  season,  in  the  fall,  so  that  in  the 
following  spring  all  that  is  necessary  is  the  expansion  of 
these  cells,  causing  an  elongation  of  the  internodes  and 
the  appearance  of  the  branch  above  ground  in  early 
spring. 
24 


370 


STRUCTURE    AND    LIFE    HISTORIES 


The  sterile  branches  produce  smaller  side  branches  at 
the  nodes.  Chlorophyll  is  formed  in  the  cells  of  the  cortex, 
and  stomata  in  the  epidermis  permit  the  exchange  of 


Fig.  267. — Equisetum  arvense.  P,  sterile  branch;  P^,  fertile  branch 
with  strobilus,  or  cone;  R,  rhizome  (underground);  T.,  cross-section  of 
cone,  showing  insertion  of  sporangiophores  in  a  whorl;  A",  iV,  sporangio- 
phores  with  pendant  sporangia;  S,  S\  S'-,  spores  with  coiled  elaters  (el). 

gases  necessary  in  photosynthesis  and  respiration.  The 
sterile  branches  elaborate  practically  all  of  the  food  neces- 
sary to  nourish  the  underground  stem  and  the  developing 


CALAMITES    AND    LYCOPODS  37 1 

sterile  and  fertile  branches  that  will  appear  the  following 
spring.  We  thus  have  an  excellent  illustration  of  the  division 
of  physiological  labor — one  branch  to  anchor  the  plant  in 
the  soil,  and  serve  as  a  storehouse  of  food  and  a  center  of 
distribution ;  roots  to  take  in  water  and  dissolved  minerals ; 
sterile  aerial  branches  to  perform  the  functions  of  food- 
manufacture,  and  the  fertile  branches  to  perform  the  func- 
tion of  reproduction — bearing  the  spores,  and  lifting  them 
high  in  the  air,  thus  facilitating  their  distribution  by  wind. 

The  fertile  branch  commonly  appears  first  in  the  spring, 
usually  bearing  no  side  branches  nor  foliage-leaves,  but 
only  whorls  of  scale-like  leaves  at  each  node.  These 
scales  possess  little  or  no  power  of  photosynthesis,  and 
are  chiefly  protective  (Fig.  267).  In  some  instances  the 
fertile  branches  bear  green  lateral  branches.  At  the 
apex  of  the  fertile  branch  is  borne  the  strohilus,  or  cone, 
consisting  of  a  central  axis  (the  prolongation  of  the 
axis  of  the  branch),  bearing  a  variable  number  of  spor- 
angiophores.  In  the  development  of  the  fertile  branch  the 
cone  is  formed  first,  and  is  raised  above  ground  by  the 
subsequent  formation  and  elongation  of  the  sterile  tissue 
below  it.  In  some  species  {E.  arvense)  the  fertile  branch 
dies  after  the  shedding  of  the  spores,  while  in  other  species 
{e.g.,  E.  pratense),  after  the  spores  are  shed  the  entire  cone 
falls  away  and  the  fertile  branch  then  takes  on  the  char- 
acters of  the  sterile  branches  which  occur  with  it. 

Each  sporangiophore^  consists  of  a  stalk  with  a  peltate 
shield  at  the  end      The  axis  of  the  cone  soon  ceases  to 

^  The  sporangiophores  of  Eqiiisetum  have  been  interpreted  as  hom- 
ologous with  leaves,  i.e.,  as  sporophylls,  but  evidence  derived  in  part  from 
a  study  of  the  fossil  relatives  of  the  modern  horsetails  indicates  that  this 
conclusion  may  not  be  correct  (Cf.  Fig.  268).  The  term  sporangiophore 
is  non-commital  as  to  homology. 


372 


STRUCTURE    AND    LIFE    HISTORIES 


elongate  between  the  whorls  of  sporangiophores,  so  that 
the  shields  occur  in  close  contact.  It  is  this  cessation  of 
growth,  in  fact,  that  produces  the  cone;  otherwise  the 
sporangiophores  would  occur  in  whorls  distributed  at 
wider  intervals  along  the  axis. 

The  sporangia  arise  from  a  single  epidermal  cell  {euspo- 
rangiate)  on  the  underside  of  the  shield;  there  are  from 
five  to  ten  on  each  shield. 


Fig.  268. — Sphenophyllum  cuneijolium,  a  fossil  species  related  to  the 
modern  horsetails.  Diagram  of  a  longitudinal  sectional  view  of  an  axis 
bearing  sporophylls  (Sph);  s,s,  sporangia;  s^,  s^,  sporangia  in  longitudinal 
section,  showing  spores,  A  vascular  bundle  enters  the  stalk  of  each 
sporangium.     Enlarged.     (Redrawn  from  Zeiller.)     Cf.  Fig.  280. 

The  spores  (which  are  green  when  ripe)  are  alike  in  size 
{homos porous),  but  they  produce  two  kinds  of  gameto- 
phytes,  male  and  female  (dioecious).  Therefore  they 
must  be  unlike  physiologically.  Under  certain  circum- 
stances,  as  already  mentioned,    (page   359),   the   female 


CALAMITES    AND    LYCOPODS 


373 


gametophytes  may  ultimately  produce  antheridia,  and 
the  male  ones,  archegonia.  It  is  of  interest  to  note  that 
some  of  the  fossil  relatives  of  the  modern  horsetails  were 
heterosporus. 

The  structure  of  the  spores  is  unusual  in  that  they  bear 
four  ribbon-like  appendages  (elaters),  formed  from  the 
outer  wall,  and  closely  coiled  around  the  spores  (Fig.  267). 


Fig.  269. — Equisetum  palustre.  Portion  of  a  male  prothallus,  bearing 
antherida;  a,  b,  c,  three  antheridia  in  successive  stages  of  development; 
a.  empty;  sp,  escaping  sperms  and  sperm-mother-cells;  c,  antheridium 
not  yet  opened;  d,  initial  stage  in  the  development  of  an  antheridium. 
X  about  70.     (After  Sadebeck.) 

These  appendages  uncoil  in  dry  air  and  recoil  with  moisture, 
with  a  sharp,  snapping  motion,  thus  rolling  the  spores 
about. 

The  distribution  of  the  spores  is  accomplished  when 
they  are  ripe,  by  the  opening  of  the  dry  walls  of  the 
sporangia.  The  shrinking  of  the  walls  gradually  forces 
out  the  spores,  and  by  the  uncoiling  and  snapping  of  the 
elaters  the  spores  become  entangled  and  held  fast  to 
each  other  in  little  fiocculent  masses.     Thus  the  complete 


374 


STRUCTURE    AND    LIFE    HISTORIES 


isolation  of  single  spores  is  prevented,  and  the  advantage 
of  this  is  recognized  at  once  when  we  recall  that  the 
prothallia  are. dioecious, 

335.  The  Gametophytes. — Under  suitable  conditions  of 
moisture  and  temperature  the  spores  begin  to  germinate, 
and  by  successive  cell-divisions  produce  the  lobed  pro- 
thallia. The  male  prothallia  are  one  cell  in  thickness, 
and  bear  the  antheridia  at  the  tips  of  the  lobes  or  on  the 
margins  (Fig.  269). 


Fig.  270. — Female  prothallus  of  Equisetum  arvense  L.  cri,  young 
archegonium;  arz,  archegoium  before  fertilization;  st,  sterile  lobe  of  pro- 
thallus; hw,  rhizoids.  Several  lobes  were  removed  in  order  to  show 
the  cushion  and  the  archegonia.  Enlarged  about  20  times.  (After 
Sadebeck.) 


The  female  prothallia  form  a  cushion  of  relatively 
thick,  spongy  tissue  (the  meristem),  and  on  this  cushion 
(as  in  all  Pteridophytes)  are  borne  the  archegonia.  In 
contrast  to  the  true  ferns,  the  archegonia  are  borne  on 
the  upper  surface  of  the  prothallus,  and  point  upward, 
in  consequence  of  being  negatively  geotropic  (Fig.  270). 
From  the  edges  of  the  cushion  numerous  thin  flaps  of 
green  tissue  form;  these  fold  over  the  cushion,  enclosing 
the  archegonia,  and  thus  retaining  the  moisture  of  dew 


CALAMITES    AND    LYCOPODS 


375 


or  rain.  This  forms  a  favorable  environment  for  the 
multiciliate  sperms,  which  are  set  free  from  the  antheridia 
of  neighboring  male  prothallia;  and  swim  to  the  arche- 
gonia,  and  down  their  neck-canals  to  the  eggs  which  they 
fertilize. 

336.  The  New  Sporophyte. — As  always,  the  fertilized 
egg  develops  into  an  embryo,  and  the  embryo,  without  any 


5a  V'       ^  "-^4^ 

Fig.  271. — Diagram  of  life-cycle  of  Equisetum. 


period  of  rest,  continues  to  grow  until  the  new  sporophyte 
is  formed,  with  underground  rhizome,  and  finally  with 
the  sub-aerial  sterile  and  fertile  branches,  thus  completing 
the  life-cycle^  (Fig.  271). 

^  In  a  few  species  of  Equisetum  modified  underground  branches,  re- 
sembling a  string  of  tubers  are  formed,  and  these  give  rise  to  new  plants  by 
vegetative  multiplication. 


376  STRUCTURE    AND    LIFE    HISTORIES 

The  life  history   of  Equisetum  may  be  tabulated  as 
follows : 

OUTLINE  OF  LIFE  HISTORY  OF  EQUISETUM 

Rhizome  i 


X.  \,  I  Sporophyte 

Sterile  branch      Fertile  branch     I 

4.4. 

Strobilus 

4-4- 

Sporangiophores 

4-4- 

Sporangia 

Reduction 
Isospore  Isospore 

4-  i 

Male   gametophyte     Female  gametophyte 

4-  4- 

Antheridium  Archegonium 

4-  4- 

Sperm  Egg 

FertilizatioiJ^ 

Oosperm 

44- 

Embryo 

4-i 

Mature  sporophyte 

11.  THE  CLUB-MOSSES  (LYCOPODIALES) 

LYCOPODIUM 

337.  Habitat  and  Distribution. — Nearly  all  the  species 
of  Lycopodium  prefer  moist  situations,  and  one  or  two  of 
them  are  aquatic.  They  are  widely  distributed  over  the 
earth,  in  both  hemispheres,  from  the  torrid  to  the  frigid 
zones,  and  commonly  grow  in  shady  places.  Lycopodium 
Selago,  and  a  few  other  species  are  epiphytic.     They  all 


CALAMITES    AND    LYCOPODS 


377 


prefer  a  substratum  rich  in  humus  or  other  organic  matter. 
Most  of  the  species  are  restricted  to  one  hemisphere,  but 
a  few  occur  in  both. 

338.  The  Sporophjrte. — There  are  several  hundred 
species  of  Lycopodium.  Among  those  most  common  in 
temperate  America  are  L.  davatum  (Fig.  272),  L.  ohscurum 


Fig.  272. — Lycopodium  davatum, 

dendroideum,  and  L.  lucidulum.  These  species  commonly 
grow  trailing  over  the  surface  of  the  ground,  and  from 
this,  and  the  appearance  of  their  foliage,  they  are  com- 
monly called  "ground  pine,"  though  of  course  they  have 
nothing  to  do  with  true  pines.  As  is  well  shown  in  the 
figure,  the  plant-body  of  the  sporophyte  of  Lycopodium 
davatum  consists  of  a  sterile  lower  region,  bearing  foliage- 


378 


STRUCTURE    AND    LIFE    HISTORIES 


leaves,  but  no  sporophylls,  while  the  fertile  region  occurs 
as  a  clearly  recognized  cone,  formed  by  the  crowding  of 
the  sporophylls  at  the  apex  of  the  leafy  axis  (Fig.  273). 
The  foliage-leaves  are  all  simple  and  small  (microphyllous) . 


Fig.  273. — Lycopodium  sp.  Photomicrograph  of  longitudinal  section 
of  a  cone,  showing  the  sporangia  on  the  upper  surface  of  the  sporophylls, 
near  their  insertion  on  the  main  axis. 


A  more  primitive  type  is  found  in  Lycopodium  Selago 
(Fig.  274).  Here  the  lower  region  is  sterile,  but  is  not 
as  well  developed  as  in  other  types,  for  the  sporophylls 
begin  to  appear  lower  down  on  the  stem.  Moreover  the 
sporophylls  are  not  aggregated  into  a  cone,  but  are  dis- 
tributed at  intervals  from  near  the  base  to  near  the  apex, 


CALAMITES    AND    LYCOPODS 


379 


with  sterile  regions  intervening.     The  leaves  usually  occur 
in  whorls  of  five,  but  often  they  are  arranged  in  spirals. 

At  the  zone  of  transition  from  sterile  to  fertile  regions, 
imperfectly  developed  [aborted)  sporangia  are  often  formed, 
and  this  (with  other  evidence)  has  suggested  that,  in  the 


Fig.  274. — Lyco podium  Selago.     (After  Bower.) 

evolution  of  the  sporophyte,  the  purely  vegetative  regions  have 
resulted  from  a  sterilization  of  fertile  tissue.  The  correctness 
of  this  interpretation  of  the  origin  of  the  sterile  regions  is 
rendered  more  probable  by  the  fact  that  the  condition 
found  in  L.  Selago  is  characteristic  of  the  fossil  Lycopods 
of  the  coal  measures.     The  possession  of  a  well-developed 


380  STRUCTURE   AND    LIFE    HISTORIES 

sterile  region  with  foliage-leaves,  and  the  restriction  of  the 
sporophylls  to  the  apices  of  the  branches  is  of  very  con- 
siderable advantage,  making  possible  an  abundant  supply 
of  food  to  a  vast  number  of  spores.  The  branching  of 
L.  Selago  is  dichotomous,  and  the  sporangia  are  borne 
on  the  upper  surfaces  of  the  sporophylls,  near  their  bases. 

339.  The  Gametophyte. — The  spores  of  Lycopods  are 
alike  in  size  (isospores),  and  on  germination  produce 
fleshy  prothallia,  bearing  both  antheridia  and  archegonia 
(monoecious),  and  partially  saprophytic  in  habit.  They 
commonly  have  a  filamentous  fungus  growing  parasitic- 
ally  within  their  tissues.  The  reproductive  organs  occur 
at  the  upper  end,  surrounded  with  sterile  hairs  {para- 
physes).  The  whole  of  the  antheridia,  and  the  venters  of 
the  archegonia  are  imbedded  in  the  vegetative  tissue. 
The  gametophyte  may  grow  more  or  less  completely  im- 
bedded in  the  soil,  but  when  growing  on  the  surface  chloro- 
phyll is  formed,  and  photosynthesis  may  take  place.  Its 
lower  end  bears  numerous  rhizoids. 

340.  The  Embryo. — After  an  ^gg  is  fertilized  it  begins 
at  once  to  divide  and  soon  develops  an  embryo.  Of  the 
two  cells  resulting  from  the  first  division  of  the  fertilized 
egg,  the  lower  one  serves  as  a  suspensor,  while  the  other 
becomes  the  ancestor  of  all  the  cells  of  the  embryo.  The 
suspensor  serves  to  push  the  embryo  down  into  the  nour- 
ishing tissue  of  the  prothallus.  The  young  embryo 
(Fig.  275)  soon  becomes  differentiated  into  two  distinct 
regions— the  foot  (not  shown  in  the  figure),  and  the  shoot. 
As  in  the  mosses  and  true  ferns,  the  foot  serves  to  absorb 
nourishment  from  the  gametophyte.  But  the  embryo  is 
dependent  upon  the  gametophyte  for  only  a  relatively 
brief  period — shorter  than  in  the  mosses  and  true  ferns. 


CALAMITES    AND    LYCOPODS 


381 


The  elongation  of  the  embryo-stem  Qiypocotyl)  carries  the 
first  leaves  {cotyledons)  up  above  the  surface  of  the  soil, 
while  at  the  same  time  the  first  root  {radicle)  is  develop- 


FiG.  275. — Lycopodium  phlegmaria.     Development  of  embryo,     st,  stem; 
cot,  cotyledon;  sus,  suspensor;  R,  root.      (After  D.  H.  Campbell.) 


Fig.  276. — Young  sporophyte  of  Lycopodium  cermmm  L.,  with  the 
gametophyte,  having  irregular  lobes  of  chlorophyll-bearing  tissue  attached 
on  one  side.     (After  Treub.) 

ing  at  its  base  (Fig.  276).  If  the  prothallus  is  deeply 
buried  the  hypocotyl  becomes  more  elongated  before  the 
cotyledons  are  formed. 

341.  Vegetative  Multiplication. — Several  species  of  Ly- 
copodium    bear    gemmae.      They    are     conspicuous    on 


382  STRUCTURE    AND    LIFE    HISTORIES 

Lycopodium  lucidulum  (which  is  common  in  the  northern 
United  States),  and  are  borne  near,  but  not  in,  the  axils 
of  the  leaves.  A  young  rootlet  commonly  appears  on  the 
gemma  while  it  is  still  attached  to  the  plant.  After  it 
falls  off,  the  axis  elongates,  and  a  new  sporophyte  is 
formed,  like  the  old  one. 

342.  Life  History. — The  student  should  be  able  to  make 
his  own  diagram  of  the  life  history  of  Lycopodium,  fol- 
lowing the  examples  given  in  connection  with  forms 
previously  studied. 

m.  LITTLE  CLUB-MOSSES  (SELAGINELLALES) 
SELAGINELLA    (LITTLE  CLUB-MOSS) 

343.  Habitat  and  Distribution. — The  little  club-mosses 
(species  of  Selaginella)  are  found  in  every  continent  and 
on  most  of  the  larger  islands.  They  usually  grow  only  in 
moist  situations,  and  are  very  common  in  conservatories, 
under  the  plant  benches,  and  in  pots  and  hanging  baskets. 

344.  The  Sporophyte. — The  plant  body  (sporophyte) 
consists  of  a  much-branched  stem  (Fig.  277),  bearing 
scale-like,  but  green  foliage-leaves,  sessile  and  more  or  less 
closely  appressed  to  the  stem.  At  the  tips  of  the  branches 
the  foliage-leaves  are  replaced  by  sporophylls,  so  arranged 
as  to  form  a  clearly  distinguished  cone  (Figs.  278  and  281). 
Near  the  base  of  the  leaves,  on  the  surface  next  the  stem, 
is  formed  a  thin,  membraneous  flap,  the  ligule  (Figs.  279 
and  280).  The  possession  of  a  ligule  is  one  of  the  funda- 
mental distinguishing  characteristics  of  Selaginella. 

345.  Two  Kinds  of  Spores.  Heterospory. — If  we 
examine  a  longitudinal  section  of  a  cone,  we  shall  normally 
find  a  sporangium  in  the  axil  of  each  sporophyll.  In 
exceptional  cases  the  lower  or  basal  leaves  of  the  cone  are 


CALAMITES   AND   LYCOPODS 


383 


Fig.  277. — Selaginella  Wildenovii, 


Fig.  278. — Selaginella  anmna.    Branch  bearing  numerous  terminal  cones. 


384 


STRUCTURE    AND    LIFE    HISTORIES 


Fig.  279. — Selaginella  Watsoniana.     Base  of  foliage  leaf,  showing  ligule, 
L.     Enlarged  about  36  times. 


Fig.  280. — Selaginella  sp.  Photomicrograph  of  a  longitudinal  section 
through  the  tip  of  a  cone,  showing  sporangia  (sp),  and  ligules  (%.). 
(Cf.  Fig.  268.) 


CALAMITES    AND    LYCOPODS 


38s 


Sterile.  The  lower  sporangia  are  larger  than  the  upper 
ones,  and  bear  four  large  spores  {megas pores).  They  are 
megasporangia,  and  the  leaves  are  megasporophylls.     The 


Fig.  281. — Sdaginella  Marlcnsil.  a,  vegetative  branch;  h,  portion  of 
the  stem,  bearing  cones  {x)\  c,  longitudinal  section  of  a  cone,  showing 
microsporangia  {mic.  sp.)  in  the  axils  of  microsporophylls,  and  megaspor- 
angia in  the  axils  of  megasporophylls;  d,  microsporangium  with  micro- 
sporophyll;  e,  microspores;/,  portion  of  wall  of  sporangium,  greatly  magni- 
fied; g,  megaspore;  h,  microsporangium  opened,  and  most  of  the  micro- 
spores scattered;  i,  megasporangium,  with  megasporophyll;  h,  same, 
opened,  showing  the  four  megaspores. 

smaller  sporangia  {microsporangia)  are  subtended  by 
viicrosporophylls,  and  bear  numerous  small  spores  {micro- 
spores).     In  some  species  the  two  kinds  of  sporophylls 

25 


386 


STRUCTURE    AND    LIFE    HISTORIES 


alternate  along  the  axis  of  the  cone  (Fig.  281).  The 
larger  number  of  microspores  results  from  the  fact  that 
every  spore-mother-cell,  by  tetrad-division,  develops 
spores,  while  in  the  megasporangia  only  one  spore-mother- 
cell  develops  spores,  the  other  cells  serving  to  nourish  that 
one.  The  microspores  develop  only  male  gametophytes, 
the  megaspores,  female.  In  dissemination,  the  spores 
are  ejected  to  some  distance  from  the  parent  plant 
(Fig.  282). 


Fig.  282. — Selaginella  Martensii.  Dissemination  of  spores.  The 
branch  was  covered  with  a  glass  bell-jar  to  avoid  currents  of  air.  The 
"dust"  is  composed  of  both  microspores  and  megaspores,  and  indicates 
the  distance  to  which  the  spores  are  projected  from  the  dehiscing  spor- 
angia.    X  23. 

346.  The  Male  Gametophyte. — The  male  gametophyte 
is  developed  entirely  within  the  wall  of  the  microspore. 
The  first  division  gives  rise  to  a  vegetative  (or  sterile) 
and  a  fertile  cell.  The  vegetative  tissue  never  develops  be- 
yond the  one-celled  stage.  By  several  divisions  the  fertile 
cell  develops  a  simple  antheridium  containing  four  sperms. 
Each  sperm  bears  two  long,  slender  cilia  (Fig.  284). 

347.  The  Female  Gametophyte. — The  megaspores  begin 
to  gerviinate  ivhile  still  in  the  sporangium.     This  will  be 


CALAMITES    AND    LYCOPODS  387 

recognized  at  once  as  a  new  feature  in  life  history.  By 
successive  divisions  the  protoplasm  of  the  spore  becomes  a 
multicellular  body,  the  prothallus,  with  richer  cells  near  the 
apex.  Here  a  number  of  archegonia  form  (Fig.  283),  and 
the  enlargement  of  the  prothallus,  or  gametophyte,  causes 
a  splitting  apart  of  the  old,  thick  walls  of  the  megaspore, 
so  that  the  female  gametophyte  protrudes  (Fig.  284,  9). 


Fig.  283. — Selaginella  Krausslana.  C,  section  of  mature  female  gam- 
etophyte, showing  three  archegonia,  two  containing  eggs,  and  one  (at  the 
left)  an  embryo  with  suspensor  {sus.).  D-G,  Stages  in  the  development 
the  archegonium;  H,  very  young  embryo  (two-celled  stage),  after  first 
division  of  the  fertilized  egg;  I,  older  embryo  (Em),  with  suspensor  {s). 
(After  Campbell.) 

It  bears  no  chlorophyll,  living  entirely  as  a  parasite  on 
the  parental  sporophyte,  from  whence  it  derives  all  the  food 
with  which  it  nourishes  the  embryo. 

348.  Fertilization, — As  throughout  the  ferns,  calami tes, 
and  lycopods,  fertilization  is  accomplished  by  the  swimming 
of  the  sperm  to  the  mouth  of  the  archegonium,  and  down 
the  neck-canal  to  the  ripe  egg  in  the  venter.  Thus  while 
Selaginella  is,  in  other  respects,  a  land-plant,  it  retains 
the  aquatic  method  of  fertilization.  External  water  is 
absolutely  necessary  in  order  that  the  sperm  may  reach 
the  egg. 

349.  The  Embryo. — After  fertilization  the  oosperm 
begins  to  divide.     The  cell  nearest  the  neck  of  the  arche- 


388 


STRUCTURE    AND    LIFE    HISTORIES 


gonium,  after  the  first  division  of  the  egg,  is  a  suspensor, 
but  becomes  much  longer  than  in  Lyco podium,  and 
thrusts  the  developing  embryo  deep  down  among  the 
nourishing  cells  of  the  gametophyte.  By  the  possession 
of  a  suspensor  the  Lycopodiales  and  Selaginellales  are 
distingiiished  from  the  Pteridophytes  and  Calamites. 

The  embryo  does  not  cease  growth,  and  pass  through  a 
resting  period,  but  continues  to  develop,  until  its  root  and 
shoot,  with  two  cotyledons,  emerge  from  the  prothallus. 


Fig.  284. — Diagram  of  life-cycle  of  Selaginella.     (Alodified  from  J.  H. 

SchafTner.) 


and  the  young  sporophyte  gradually  becomes  established 
as  an  independent,  green  plant  (Fig.  284).  It  is  only  after 
the  development  of  a  vigorous  leafy  shoot,  with  chloro- 
phyll apparatus,  capable  of  elaborating  an  abundance  of 
food,  that  the  strobilus  is  organized  with  its  axis  and 
green  sporophylls. 


CALAMITES    AND    LYCOPODS  389 

OUTLINE  OF  LIFE  HISTORY  OF  SELAGINELLA 

Sporophvtc 

TT  '      TT 

Microsporophyll  ^Nlegasporophyll 

^  >l^  >!<  >K 

Microsporangium  Megasporangium 

^      < —  Reduction — >       ^ 

Microspore  ]Megaspore 

4-  4. 

Male  gametophyte        Female  gametophyte 

4.  i 

Antheridium  Archegonium 

4.  4- 

Sperm  Egg 

Fertilization 


Oosperm 

4-4- 

Emhrvo 

ii 

Mature  sporophyte 

350.  Marks   of   Progress. — With   the   introduction   of 

heterospory  we  recognize  a  distinctly  new  feature  of  the 
sporophyte  generation.  Structural  differentiations  asso- 
ciated with  difference  in  sex  have  hitherto  been  confined 
to  the  gametophytic  generation,  but  now  such  distinctions 
appear  for  the  first  time  in  the  sporophyte.  This  is  a 
long  step  forward,  and  marks  Selaginella  as  a  more  highly 
organized  form  than  the  lycopods,  horsetails,  and  ferns. 
Other  marks  of  progress  are: 

1.  The  reduction  of  the  vegetative  tissue  of  the 
gametophytes  (to  only  one  cell  in  the  case  of  the  male 
gametophyte). 

2.  The  entire  dependence  of  the  gametophytes  upon  the 
sporophytes  for  nutrition. 

3.  The  retention  of  the  female  gametophyte,  through- 
out its  entire  existence,  almost  entirely  within  the  wall 
of  the  megaspore. 


CHAPTER  XXV 

SEED-BEARING  PLANTS 

THE  CYCADS 

351.  Description. — The  cycads,  while  native  only  in 
the  tropics,  are  familiar  to  all  persons  who  have  visited 
conservatories.  One  of  the  commoner  species  in  cultiva- 
tion (Cycas  revoJuia)  is  often  labelled,  "Sago  palm."     In 


Fig.  285. — Cycas   rcvolula,  showing  terminal  bud  of  foliage-leaves  just 
opening.     (Compare  Fig.  286.) 

fact,  in  some  respects  it  bears  a  superficial  resemblance  to 
a  palm,  while  in  other  characters  it  suggests  the  ferns. 
There  is  a  short,  thick,  cylindrical,  unbranched  stem  or 
trunk,  bearing  a  crown  of  beautiful,  leathery,  green  leaves, 

390 


SEED-BEARING   PLANTS 


391 


having  prominent  midribs  and  pinnately  divided  (Fig. 
285).  The  leaves  endure  for  a  year  or  more  (varying 
with  the  species),  and  are  then  replaced  by  a  fresh  crown. 
The  duration  of  each  crop  of  leaves  is  said  to  vary  accord- 
ing as  the  plant  grows  wild,  or  in  botanic  gardens  and 


Fig.  286. — Cycas  revoliita.  Terminal  bud  of  foliage  leaves  just  opening. 
Nearer  view  of  Fig.  285.  Note  the  circinate  vernation  of  the  leaf-pinnules, 
but  not  of  the  entire  leaf. 


conservatories.  Thus,  when  temperature  and  rainfall  are 
excessive,  Cycas  circinalis  may  produce  two  crowns  of 
leaves  a  year,  instead  of  the  one  crown  commonly  produced 
in  green  houses. 

The  young  leaves  are  curled  up  at  the  tips,  unrolling 
as  they  grow.     In  the  genus  Cycas  only  the  leaflets  are 


392 


STRUCTURE    AND    LIFE    HISTORIES 


r- 

J^U^ 

i 

Mk 

1 

1^^^  ''J           ^Ih 

\!^^  '^.    ^^^^k 

« 

^t> 

Fig.  287. — Macrozamia   Moorei.     Two  staminate  cones;   the  older  one 

above. 


FiG._  288— Macrozamia  Moorei.     Microsporophyll  (lower  surface),  show- 
ing microsporangia  (pollen-sacs),  after  the  shedding  of  the  pollen. 


J 


SEED-BEARING    PLANTS 


393 


curled  (Fig.  286),  while  in  the  related  American  genus, 
Zamia,  the  entire  leaf  is  curled  as  well.  The  plants  are 
dioecious,  but  the  two  sexes  so  closely  resemble  each  other 
that  they  cannot  be  distinguished  except  by  their  sporo- 
phylls  when  in  fruit. 


Fig.  289. — Macrozamia   Moorei,   showing   two   lateral   carpellate   cones. 

362.  Microsporophylls.— The  microsporophylls  are 
grouped  into  a  cone  (Fig.  287).  This  means  that  they 
are  not  on  the  main  stem,  for  the  cone  is  really  a  branch, 
bearing  only  sporophylls.  The  microsporangia,  bearing 
microspores,  occur  in  groups  (sori),  on  the  under  surface 
of  the  sporophylls  (Fig.  288). 


394 


STRUCTURE    AND    LIFE    HISTORIES 


SEED-BEARING    PLANTS 


395 


353.  Megasporophylls. — The  niegasporophylls,  or  car- 
pels, occur  either  in  axillary  cones  (Fig.  289),  or  in  groups 


Fig.  291, — Cycas  media.  Trunk,  with  foliage  leaves  removed,  showing 
crown  of  megasporophylls  (carpels).  (Specimen  in  Brooklyn  Botanic 
Garden.)     (Cf.  Fig.  289.) 

at  the  summit  of  the  main  stejn,  surrounding  the  growing 
point  of  the  stem,  and  surrounded  by  the  foliage-leaves 
(Figs.  290  and  291) .     In  the  latter  case  they  possess  a  pin- 


396 


STRUCTURE    AND    LIFE   HISTORIES 


nately  divided  blade,  with  mid-rib  and  petiole  (Fig.  292). 
The  genera  that  bear  the  megasporophylls  on  the  main 
stem  resemble  the  ferns,  and  in  this  respect  are  the  simplest, 
or  most  primitively  organized,  of  all  living  seed-plants. 

364.  Megasporangia. — Unlike  the  microsporangia,  the 
megasporangia  of  Cycas  occur,  not  in  groups,  but  solitary 


Fig.  292. — Young  megasporophyll  (carpel)  of  Cycas  revolnta,  bearing 
six  young  ovules,  destined,  after  fertilization,  to  mature  into  seeds.  Note 
the  relatively  large  amount  of  leaf-blade  above  the  ovules,  as  compared 
with  Cycas  media  (Fig.  293).     (Specimen  from  C.  C.  Chamberlain.) 

on  the  lower  part  of  the  sporophyll,  at  the  margin,  occupy- 
ing the  position  of  the  pinnate  divisions  (Figs.  292-294). 
In  genera  bearing  carpellate  cones  the  megasporangia 
occur  in  pairs  on  the  under  surface  of  each  scale  (mega- 


SEED-BEARING    PLANTS 


397 


sporophyll,  Fig.  295).     In  the  Cycads  and  higher  plants 
the  megasporangia  are  called  ovules. 

355.  Ovules. — The  young  megasporangia  or  ovules  of 
Cycads  consist  of  two  distinct  regions  of  sterile  tissue — an 
inner   nucellus,    and   an   outer   covering,    or   integument^ 


Fig.  293. — Megasporophyll  (carpel)  of  Cycas  media,  bearing  one  ripe 
naked  (gymnospermous)  seed,  and  three  ovules  which  failed  to  become 
seeds,  doubtless  through  not  being  fertilized.     (Compare  Fig,  292.) 

which  is  an  outgrowth  of  the  ovule  just  below  the  nucellus. 
The  integument  serves  to  protect  the  more  delicate  tissues 
within,  and  later  becomes  transformed  into  the  seed- 
coat  (Figs.  296-298).  Only  one  of  the  four  megaspores 
develops  within  the  nucellus. 

356.  Female  Gametophyte. — As  in  Selaginella,  the  mega- 
spores begin  to  germinate  while  still  in  the  sporangium, 
but  now  a  new  feature  is  introduced  into  life  history; 


398 


STRUCTURE    AND    LIFE    mSTORTES 


Fig.  294. — Cycas    media.     Carpel    (megasporophylD    with    six    ripened 
ovules  or  seeds. 


P'iG.  295. — Macrozamia  Moorei.  A  scale  (megasporophyll)  from  a 
carpellate  cone,  bearing  two  ovules,  maturing  into  seeds.  A,  top  view; 
5,  bottom  view.     About  half  size. 


SEED-BEARING    PLANTS  399 

Ihe  megaspores  of  Cycads  never  leave  the  sporangium.     The 
developing  female  gametophyte  is  nourished  entirely  as 


Fig.  296. — Ovule  oi  Macrozamia  Moorei.  A,  external  view;  B,  portion 
of  integument  removed,  showing  egg-shaped  gametophyte.  The  dark 
strip  on  the  upper  left-hand  portion  of  the  latter  is  where  a  piece  of  the 
now  membranous  nucellus  was  peeled  off;  C,  gametophyte,  entire,  with 
nucellar  cap  at  the  upper  end;  D,  longitudinal  section;  E,  end  view,  with 
part  of  the  integument  removed,  disclosing  the  tiny  crater-like  pollen- 
chambers,  at  the  bottom  of  which  the  necks  of  the  archegonia  open.  (Cf . 
Fig.  297.)     About  natural  size. 

a  parasite,   by  the   tissue  of  the  nucellus.      The  latter 
becomes  almost  entirely  consumed,  so  that  when  a  longi- 


400 


STRUCTURE    AND    LIFE    HISTORIES 


tudinal  section  is  made  of  a  mature  ovule  (Fig.  297),  tlie 
remainder  of  the  nucellus  appears  only  as  a  thin  mem- 
brane adhering  to  the  outer  surface  of  the  prothallus,  or 
endosperm,  as  it  is  here  called.  When  the  Qgg  fails  to 
become  fertilized  the  gametophyte  may  protrude,  develop 
chlorophyll,  and  lead  a  brief,  semi-independent  existence. 


■  i 

^^M 

mi 

.--4 

^^^^ 

W"'     ^ 

^^^^^^S^.  _»* — ■ 

-  A-T  -  —  3,  r. 

i 

t 

■ 

1 

oi..-m 

^ 

W.\i. 

1 

fc. 

*»-— — w' 

^ 

_^     «■•         ^^r 

Sti:. L-^ ■ 

■  - -  -^.^-'^-^ 

Fig.  297. — Photograph  of  a  longitudinal  section  of  an  ovule  of  Macro- 
zaniia  Moorei.  mi,  micropyle;  n.c,  nucellar  cap;  a.r,  archegonia  (the 
venters  only  showing) ;  il,  inner,  hard  layer,  o.l,  outer,  fleshy  layer  of  the 
integument;  g,  gametophyte;  p.c,  pollen  chamber.  Enlarged  from  D, 
Fig.  296. 


Several  archegonia  develop  in  the  apical  end,  imbedded 
in  the  tissue  of  the  prothallus,  and  with  their  neck-canals 
opening  at  the  surface  into  a  pollen-chamber  (Fig.  297). 
They  resemble  somewhat  the  archegonia  of  the  lower  orders 


SEED-BEARING    PLANTS 


401 


studied,  but  possess  only  two  neck  cells,  and  no  neck-canal 
cells.  The  large  egg-cell  in  the  venter  is  the  largest  known 
in  the  plant  kingdom. 


Fig.  298. — Cycas  circinalis.  Diagram  of  longitudinal  section  of  a 
nearly  mature  seed;  0,  outer  fleshy  layer,  with  a  bundle  (0^)  of  the  outer 
vascular  system;  s,  stony  layer  of  integument;  i,  inner  fleshy  layer,  with 
a  bundle  {i^)  of  the  inner  vascular  system;  c,  central  vascular  bundle. 
(After  Marie  C.  Stopes.) 


357.  Male  Gametophyte. — The  germination  of  the 
microspore  and  the  development  of  the  male  gametophyte 
involve  only  cell-divisions,  but  not  the  growth  of  new 
tissue.  The  mature  gametophyte  is  called  a  pollen- grain. 
It  consists  of  three  cells:  a  prothallial  cell,  a  tube-cell, 
and  a  generative  cell  (Fig.  299).  There  is  no  structure 
that  can  be  positively  identified  as  an  antheridium,  unless 
the  prothalHal  cell  is  considered  (as  by  some),  as  repre- 
senting the  antheridium.  The  pollen-grain  has  two 
coats — an  outer  and  an  inner. 

358.  Pollination. — When  the  pollen  is  mature  it  is 
scattered  by  the  wind,  and  some  of  the  grains  lodge,  by 

26 


402 


STRUCTURE   AND    LIFE   HISTORIES 


chance,  in  the  pollen-chamber  of  the  ovules  on  neigh- 
boring female  plants.  The  transfer  of  pollen  to  the  female 
plant  is  pollination. 

359.  Germination  of  the  Pollen-grain. — In  the  pollen- 
chamber   the    conditions    favor   the   germination    of   the 


Fig.  299. — Cycas  rcvoluta.  a,  Pollen  grains  at  shedding  stage;  X  500; 
i,  later  stage,  showing  prothallial  cell  (/>)  and  generative  cell  (g),  the  tube- 
nucleus  not  shown;  X  200;  c,  generative  cell  divided,  giving  rise  to  stalk- 
and  body-cells;  X  500;  d,  the  stalk-cell-nucleus  (5)  being  crowded  out, 
and  blepharoplasts  appearing  in  the  body  cell  ih);  X  500;  ^>  the  body-cell 
shortly  before  division,  showing  two  well-developed  blepharoplasts;  X  750; 
/,  the  two  male  cells  resulting  from  the  division  of  the  body-cell;  the  beaks 
of  the  nuclei  are  attached  to  the  cilia-bearing  bands;  X  200.  Reduced 
about  two-thirds  in  reproduction.     (After  Ikeno.) 

pollen.  This  is  also  a  new  feature  in  life  history.  In  ger- 
mination the  pollen-grain  develops  absorbing  organs 
{haustoria),  which  penetrate  the  tissue  of  the  nucellar-cap 
(Fig.  300),  and  also  larger  tubes  which  carry  the  generative 
cell  further  down  into  the  pollen-chamber.     As  the  tube 


SEED-BEARING    PLANTS 


403 


elongates  the  generative  cell  passes  down  it,  becoming 
divided  into  two  sperm-cells.  These  sperm-cells  are 
remarkably  interesting  little  bodies,  bearing  a  large  num- 


T^^  ^^^ 


Fig.  300. — Dioon  cdiile:  upper  part  of  an  ovule  at  the  time  of  fertiliza- 
tion, showing  integument,  nucellus,  male  gametophytes  (germinating 
pollen-grains),  and  female  gametophytes  (embryo-sacs).  Reconstructed 
from  sections  of  several  ovules.     (After  Chamberlain.) 

ber  of  cilia,  by  the  motion  of  which  they  are  carried,  in 
the  completion  of  their  journey  to  the  egg,  through  the 
watery  solution    emptied    from    the    pollen-tube,  to  and 


404 


STRUCTURE    AND    LIFE    HISTORIES 


through  the  neck-canal  of  the  archegonium,  into  the 
archegonial  chamber.  FertiHzation,  here  as  always,  is 
completed  by  the  fusion  of  the  sperm  and  the  egg-nuclei 
(Fig.  301).  The  behavior  of  these  little  sperms  in  an  allied 
genus,  Zamia,  is  thus  described  by  their  discoverer,^ 
Webber: 


Fig.  301. — Fertilization  in  Zamia  floridana.     The  male  and  female  nuclei 
are  fusing;  h,  remains  of  the  cilia  of  the  sperm.     (After  Webber.) 

"In  removing  the  nearly  mature  pollen-tubes  the  sperm- 
atozoids  are  found  to  be  in  various  stages  of  development, 
as  would  be  expected.  In  many  cases  tubes  have  been 
observed,  before  cutting  them  off,  in  which  the  two  sper- 
matozoids  had  pulled  apart  and  were  swimming  free  in  the 
protoplasm.  In  some  instances  their  movement  in  the 
pollen-tube,  before  it  is  injured,  can  be  observed  with  the 
aid  of  a  hand  lens. 

''It  is  an  interesting  sight  to  see  the  two  giant  sper- 
matozoids  moving  around  vigorously  in  the  pollen-tube, 

1  ^Motile  sperms  were  discovered  in  Cycas  in  1897,  by  a  Japanese  botanist , 
Hirase,  following  their  discovery  in  Ginkgo  in  1896  by  another  Japanese 
student,  Ikeno.  The  latter  was  the  first  discovery  of  motile  sperms  in  a 
spermatophyte.. 


SEED-BEARING    PLANTS  405 

bumping  against  each  other  and  the  wall  of  the  tube  in 
their  reckless  haste.  They  seldom  escape  from  the  upper 
cut  end  of  the  pollen-tube,  although  they  as  frequently- 
swim  toward  this  end  of  the  tube  as  the  other  end,  so  far 
as  could  be  observed.  In  many  cases  the  pollen-tubes 
were  cut  so  that  the  spermatozoids  escaped  into  the  solu- 
tion, and  in  numerous  other  cases  mature  turgid  tubes 
burst  in  the  process  of  cutting,  discharging  the  uninjured 
spermatozoids  in  the  sugar  solution.  The  writer  was 
thus  able  in  many  cases  to  study  the  spermatozoids  swim- 
ming free  and  observe  their  unobstructed  motion. 

''The  motion  of  the  spermatozoids  when  swimming  free 
in  sugar  solution  is  in  no  way  different  from  their  motion 
when  in  the  pollen-tube.  The  general  motion  is  a  con- 
tinuous rotation  of  the  body,  always  in  the  same  direction, 
around  an  axis  passing  through  the  apex  of  the  helicoid 
spiral.  Viewed  from  the  head  end  or  apex  of  the  spiral 
the  rotation  is  in  the  direction  of  the  hands  of  a  clock,  and 
contrary  to  the  turns  of  the  spiral  band.  They  roll 
around,  first  here,  then  there,  resembling  in  this  respect 
the  motion  of  Pandorina.  After  moving  about  rapidly 
for  from  five  to  fifteen  minutes  they  usually  cease  all  pro- 
gressive motion,  but  continue  to  rotate  for  a  considerably 
longer  period.  The  rotary  motion  also  soon  ceases,  but  the 
cilia  continue  to  vibrate  for  a  considreably  longer  time. 
The  spermatozoids  of  Zamia  also  have  an  amoeboid  mo- 
tion, which  is  particularly  noticeable  while  they  are  in- 
closed in  the  pollen-tube.  The  apex  of  the  spiral  as  a 
whole  frequently  rotates  in  a  most  remarkable  way, 
turning  in  a  circle,  pushing  out  first  this  way  and  then  that 
way  with  the  greatest  freedom  of  motion,  as  if  selecting  a 
point  of  exit  or  ingress.     In  other  cases  the  base  or  the  side 


4o6  STRUCTURE    AND    LIFE    HISTORIES 

of  the  spermatozoid  body  may  be  considerably  extended 
as  a  blunt  point  in  pushing  between  two  obstacles.  The 
whole  body  seems  flexible  and  changeable  in  the  highest 
degree  and  is  eminently  fitted  for  its  difficult  task  of  finding 
and  swimming  through  the  narrow  passage  between  the 
neck-cells  of  the  archegonia." 

A  point  to  be  specially  noted  here,  is  that  while  a  pollen- 
tube  is  introduced  in  the  process  of  fertilization,  the  final 
act  is  accomplished  as  in  lower  aquatic  forms,  by  the 
swimming  of  the  sperms  through  liquid.  The  pollen- 
tube  alone,  as  in  higher  plants,  should  suffice  in  Cycads  to 
bring  the  sperm  to  the  egg,  and  the  retention  of  locomotion 
of  the  sperm,  after  the  appearance  of  the  pollen-tube,  can 
be  interpreted  only  as  the  persistence,  by  inheritance,  of  a 
character  that  was  a  fundamental  necessity  in  lower 
forms.  ^ 

360.  The  Seed. — During  the  processes  of  germination  of 
the  pollen-grains  and  fertilization,  the  ovule  is  increasing 
in  size,  and  developing  different  tissues  and  juices.  The 
outer  wall  of  the  nucellus  hardens,  while  the  integument 
becomes  succulent  and  pulp-like,  so  that  externally  the 
structure  resembles  a  plum.  It  cannot,  however,  be  com- 
pared to  a  plum  in  morphology  {i.e.j  cannot  be  homologized 
with  a  plum),  for  a  plum  is  a  ripened  ovary,  while  the 
so-called  ''fruit"  of  the  Cycads  is  a  ripened  ovule  or 
seed. 

361.  The  Embryo. — After  fertilization  the  oosperm 
develops  into  an  embryo-sporophyte  (P'ig.  302).  This  is 
often  delayed  until  after  the  seed  is  planted,  so  that  after 
the  embryo  has  once  begun-  to  form  it  continues  to  grow, 

^  The  accomplishment  of  fertilization  by  the  mediation  of  a  pollen-tube 
(siphon)  is  called  siphonogajny. 


SEED -BEARING    PLANTS 


407 


without  undergoing  any  resting  period,  until  the  new  sporo- 
phyte  has  emerged  from  the  seed  and  become  established 


;.'  d 


.      'o,ao® 


/ 


J 

1 


c   t 


Fig.  302. — Zamia  jloridana.  a,  free  nuclei  of  proembryo;  X  16;  b, 
tissue  at  base  of  proembryo;  X  24;  c,  diflferentiation  into  suspensor  and 
embryo;  X  29;  d,  young  embryo  showing  long  suspensor,  natural  size. 
(After  Coulter  and  Chamberlain.) 


in  the  soil  as  an  independent  plant  (Fig.  303).  In  its  early 
stages  the  embryo  derives  its  nourishment  as  a  parasite 
from  the  female  gametophyte  (prothallus,  or  endosperm). 


4o8 


STRUCTURE    AND    LIFE    HISTORIES 


362.  Gymnospermy.-  The  fact  that  the  seed  is  not 
enclosed  within  the  carpels,  or  walls  of  the  ovary,  but 
continues  wholly  exposed  throughout  its  existence,  until 


Fig.  303. — Cycas  incdia.  Germinating  seeds,  e,  endosperm  (gameto- 
phyte);  c,  cotyledons;  st,  leaf-stalk;  h,  hypocotyl,  showing  early  enlarge- 
ment to  form  the  thick  trunk  shown  in  Fig.  291. 


shed,  is  one  of  the  most  significant  of  all  the  features  of 
the  cycads.  On  this  feature,  and  its  opposite,  the  great 
division  of  seed-bearing  plants  (Spermatophytes)  is  sepa- 
rated into  two  classes  as  follows: 

„  .1.1  Gymnosperms 

Spermatophytes  <    .      . 

[  Angiosperms 

The  word   gymnosperm   means   ''naked   seed,"^  while 

^  From  the  Greek,  gymnos  {yviivbs),  naked  -f-  spernta  {airkp/jia),  seed. 


SEED-BEARING    PLANTS  409 

angiosperm  means  "enclosed  seed."^  The  fundamental 
distinction  between  gymnospermy  and  angiospermy  was 
first  made  clear  by  one  of  the  greatest  of  English  bota- 
nists, Robert  Brown,  in  1827  (Fig.  10). 

363.  Comparison  with  Ferns. — The  cycads  resemble  the 
true  ferns  in  several  points — in  the  vernation  of  their 
leaves  (coiled  in  the  bud),  in  the  venation  of  the  leaves 
(forked  veins),  in  the  possession  of  sori  (for  microspor- 
angia),  in  having  multiciliate  sperms,  in  having  sporo- 
phylls  (megasporophylls)  that  closely  resemble  foliage 
leaves,  and  in  having  the  embryo  dependent  at  first  upon 
the  prothallus  for  nourishment,  but  later  becoming 
established  as  an  independent  plant. 

They  differ  from  ferns  in  having  the  non-green  gameto- 
phyte  dependent  for  nourishment  throughout  its  life 
upon  the  tissues  of  the  sporophyte.  In  the  heterosporous 
habit  they  differ  from  the  true  ferns,  but  resemble  the 
higher  fern  relatives,  like  Selaginella.  Their  greatest 
step  forward  is  the  development  of  a  seed.  They  are  the  first 
true  seed-bearing  plants  to  be  met  with  among  living 
species,  as  we  ascend  from  the  Algae.  How  closely 
Selaginella  approaches  the  formation  of  a  true  seed  may 
be  seen  by  referring  to  the  condition  in  the  megaspor- 
angium  following  fertilization  (Fig.  284).  If  the  female 
gametophyte  of  Selaginella  should  remain  within  the  walls 
of  the  megaspore;  if  the  embryo  should  undergo  a  period  of 
rest  after  the  formation  of  the  young  stem  and  first  leaves, 
and  if  this  entire  structure  should  remain  within  the  mega- 
sporangium,  we  should  have  a  true  seed  in  Selaginella. 

The  cycad  seed  is  primitive  (or  imperfect  as  a  seed), 

^  From  the  Greek,  angcion  {a-yyeiov),  a  vessel  -}-  sperma  {a-irkpua), 
seed. 


4IO  STRUCTURE    AND    LIFE    HISTORIES 

in  frequently  not  having  the  embryo  develop  until  after 
the  seed  is  planted,  and  in  not  having  the  embryo  undergo 
a  period  of  rest  between  its  formation  and  its  final  develop- 
ment into  a  mature  sporophy te ;  the  absence  of  an  embryo 
characterizes  some  of  the  fossil  relatives  of  Cycas, 

The  genus  Cycas  is  the  only  living  genus  of  plants  which 
produces  seeds  w^ithout  developing  either  a  cone  or  a  flower, 
but  produces  the  seeds  on  sporophy  Us  borne  on  I  he  main 
stern.  It  is  the  simplest  of  all  living  seed-plants,  to  this 
extent,  being  in  this  respect  as  lowly  organized  as  the 
ferns. 

OUTLINE  OF  LIFE  HISTORY  OF  A  CYCAD 

The  Cycad  plant 
(Sporophyte) 

Microsporophylls  Megasporophylls 

>l^  >!'  ■A'  -^ 

Microsporangia  Megasporangia  (ovules) 

4,  ^ — Reduction — >         \. 

Microspores  Megaspores 

4,  4. 

Male  gametophyte  Female  gamctophyte 

(Pollen-grain)  (Endosperm) 

i  i 

Vestigial  antheridium  Archegonium 

Generative  cell 

4. 

Sperms  Egg 

Fcrtilizahuii 


Oosperm 

a 

Embryo 

New  Cycad  plant 
(Sporophyte) 


;on 

1 


SEED-BEARING    PLANTS 


411 


364.  Distribution. — The  cycads  and  their  relatives  are 
all  tropical  or  subtropical.  Four  genera  {MicrocycaSy 
Zamia,  Ceratozamia,  Dioon)  occur  in  the  western  hemi- 
sphere, with  the  chief  center  of  distribution  in  Mexico; 
and  five  genera  {Cycas,  Bowenia,  Macrozamia,  Encephal- 
artos,   and   Stangeria)    in   the   eastern   hemisphere,   with 


Fig.  303a. — Wilhelm  Hofmeister  (1824-1877).  His  comparativ^e  studies 
of  the  history  of  development  of  mosses,  vascular  cryptogams,  and  seed- 
bearing  plants,  disclosed  the  fact  of  an  alternation  of  generations  through- 
out those  forms,  and  afforded  the  basis  for  a  correct  interpretation  of  the 
genetic  relationship  of  the  great  groups  of  plants. 


the  chief  center  of  distribution  for  the  first  three  in  Queens- 
land, Australia.  Encephalartos  and  Stangeria  are  confined 
to  Africa. 


CHAPTER  XXVI 

SEED-BEARING  PLANTS  (Continued) 

GYMNOSPERMS 

LIFE  HISTORY  OF  THE  PINE 

Description  of  the  Tree 

365.  The  Trunk. — Everyone  is  so  familiar  with  the 
general  features  of  pine  trees  as  to  render  a  detailed  de- 
scription unnecessary  here.  The  main  stem  or  trunk  is 
normally  a  straight  vertical  shaft,  that  may  be  traced  from 
the  ground  to  the  apex  of  the  tree  (Fig.  304).  Such  a 
trunk  is  called  excurrent,  in  contrast  to  the  opposite  type 
which  may  be  traced  for  only  a  short  distance  from  the 
ground,  to  a  point  where  it  divides  into  the  main 
branches  or  limbs.  The  latter  type  of  trunk  is  termed 
deliquescent  (Fig.  305). 

The  excurrent  trunk  results  from  the  fact  that  the 
main  stem,  as  well  as  the  lateral  branches,  always  has  a 
terminal  hud,  which  carries  the  trunk  upward  from  year 
to  year.  Trees  may  continue  to  increase  in  height  each 
season,  until  they  have  reached  the  limit  for  the  given 
species.  Many  factors  may,  of  course,  modify  the  height, 
among  which  is  the  limit  of  height  to  which  the  species 
can  raise  sap.  Some  species  of  pine  attain  a  height  of 
160  feet. 

366.  Branching. — At  first  glance  pine  trees  appear  to 
bear  their  main  branches  in  circles  or  whorls,  but  closer 

412 


SEED-BEARING   PLANTS 


413 


lie.  304.  -Glove  ui  puici  bliowing  excurrent  trunks. 


414 


STRUCTURE    AND    LIFE    HISTORIES 


observation  discloses  the  fact  that  these  are  really  pseudo- 
whorls,  since  the  branches  do  not  emerge  at  precisely  the 
same  level  on  the  trunk,  as  in  true  whorls.  Both  because 
they  are  older,  and  because  they  receive  less  light,  the 


Fig.  305. — Elm   tree    {Ulnms  americana),   showing  deliquescent    trunk. 

lower  branches  are  longer,  and  the  gradual  decrease  in 
length  from  below  upward,  combined  with  the  excurrent 
habit  of  the  trunk,  gives  the  tree  a  pyramidal  outline  when 
it  grows  free  in  the  open.     This  shape  is  modified  by 


SEED-BEARING    PLANTS  415 

mutual    shading    and    crowding,    resulting    in    natural 
pruning,  when  trees  grow  close  together  in  the  forest. 

367.  Long  and  Short  Branches. — The  pines,  like  several 
other  groups  of  plants,  bear  two  distinct  kinds  of  branches 
— long  branches,  and  short  branches.  The  short  branches 
are  often  called  "  spur-shoots. 'V  Long  branches,  in 
addition  to  being  longer  and  larger,  commonly  bear 
only  scale-like  leaves,  while  the  spur-shoots  bear  the 
foliage-leaves  (Fig.  308).  In  certain  cases  juvenile  forms 
of  long  branches  are  recognized,  which  bear  foliage-leaves 
as  an  exception  to  the  general  rule. 

368.  Leaves. — The  long,  needle-like  leaves  of  the  pine 
are  familiar  to  everyone.  They  occur  on  the  spur-shoots 
in  groups  or  fascicles  of  one,  two,  three,  or  five,  according 
to  the  species.  The  spur-shoots  are  borne  in  the  axils  of 
the  scale-like  leaves  of  the  long  shoots  (Frg.  307).  The 
base  of  the  leaf  cluster  is  sheathed  by  the  thin  membranous 
scales  of  the  terminal  bud  of  the  spur.  When  there  are  two 
leaves  in  a  fascicle  they  are  semi-circular  in  cross-section, 
the  adjacent  faces  being  flattened  as  a  result  of  their 
contact  in  the  bud;  when  there  are  three  or  more  they 
are  triangular  in  cross-section.  The  white  pine  {Pinus 
Strobus),  and  its  nearest  relatives,  have  five  leaves  to  a 
fascicle,  the  pitch-pine  (P.  rigida),  and  its  nearest  relatives, 
bear  the  leaves  in  threes,  the  scrub-pines  and  the  Euro- 
pean pine  {P.  sylvestris),  in  twos. 

369.  Leaf -fall. — The  duration  of  the  leaves  varies  with 
the  species  and  the  locality  from  two  to  ten  years.  As  a 
result  there  are  always  green  leaves  on  the  tree,  whence  the 
term  '' evergreen."  That  the  leaves  are  shed  may  be 
easily  determined  by  examining  the  ground  under  any 
pine    tree.     The   duration   of   the   leaves   is   also   easily 


4i6 


STRUCTURE    AND    LIFE    HISTORIES 


ascertained  by  observing  the  oldest  annual  growth  still 
bearing  spur-shoots.  When  the  leaves  are  shed  the 
entire  spur-shoot  falls  away. 

Reproduction 

370.  Staminate  Cones. — Most  of  the  cone-bearing  trees 
(Coniferae)  are  monoecious,  i.e.,  bear  both  microspores 
and  megaspores  on  the  same  tree.  The  staminate  cones 
appear  in  the  spring,  usually  in  May  in  the  northern  states, 


Fig.  306. — Staminate    cones    of    the    Austrian    pine    {Pinus    auslriaca). 
Below,  before  shedding  pollen;  above,  after  shedding. 

and  persist  for  only  a  few  weeks.  They  are  borne  in 
clusters  on  the  long  branches  on  the  current  year's  growth, 
and  occupy  the  lateral  position  of  spur-shoots;  they  are 
never  terminal  (Fig.  306).     They  are,  in  reality,  modified 


SEED-BEARING    PLANTS 


417 


branches,  consisting  of  the  main  axis,  bearing  the  scales 
(microsporophylls,  or  stamens)  arranged  in  spirals.  On 
the  underside  of  the  microsporophyll  are  the  spore-cases, 
containing  the  microspores.  The  staminate  cones  pass  the 
winter  inside  the  bud,  and  during  this  period  the  sporangia 
contain  only  spore-mother-cells.     The  tetrad-division  of 


Fig.  307. — Scotch  pine  {Piniis  sylveslris).  Showing  carpotropism  of 
carpellate  cones.  Young  cones  at  the  left;  cones  one  year  old  at  the  right. 
(Cf.  Figs.  308  and  309.) 

the  spore-mother-cells,  occurs  early  in  the  spring,  the  exact 
time  varying  with  the  species,  the  locality,  and  the 
character  of  the  season,  but  the  young  microspores  are 
usually  all  formed  by  the  first  of  May.  On  either  side 
of  the  main  body  of  the  microspore  is  a  tiny  air-sac  which 
gives  the  spore  great  buoyancy  in  the  air  (Fig.  313). 
27 


4i8 


STRUCTURE    AND    LIFE    HISTORIES 


371.  Carpellate  Cones. — One  or  more  young  carpellate 
cones  appear  near  the  tip  of  the  new  growth  in  early 
sprinc:   fFigs.   307-309),   and  are  noticeable  at  that  time 


1 

^•^ 

v-<\ 

\\A  \i\   , 

/ 

\ 

,^ 

Ail 

1 

^ 

& 

W 

^.^/gmfM. 

t 

s 

^  ^ 

Jl 

IK 

fl 

3 

lli 

W 

V 

/1j\                     \ 

\ 

M 

\  ^ 

/K 

/ 

Fig.  308. — Scotch    pine    (Pinus   sylveslris).     Branch    bearing   carpellate 
cones,  one  month,  one  year,  and  two  years  old.     (Cf.  Figs.  307  and  309.) 


from  their  delicate  tint  of  red.  They  terminate  short, 
lateral,  axillary  branches.  Like  the  staminate  cones,  the 
carpellate  cones  are  branches,  modified  for  the  purpose  of 


SEED-BEARING    PLANTS 


419 


reproduction.  The  central  axis  bears  lateral  scales  (Figs. 
309  and  310),  but  the  homology  of  these  scales  is  difficult 
to  determine,  and  is  still  a  matter  of  debate.     There  are 


Fig.  309. — Scotch  pine  {Finns  sylvestris).  A-D,  stages  in  the  develop- 
ment of  the  carpellate  cone,  and  its  carpotropic  movements.  E,  very 
young  carpellate  cone  much  enlarged;  F,  ventral,  G,  dorsal  views  of  a 
scale  from  E;  1,  ovuliferous  scale;  2,  ovule  (in  longitudinal  section);  3, 
pollen  chamber  and  micropyle  leading  to  the  apex  of  the  nucellus  (mega- 
sporangium);  4,  integument  of  the  ovule;  G,  i,  tip  of  ovuliferous  scale; 
5,  bract;  4,  integument;  //,  longitudinal  section  at  right  angles  to  the 
surface  of  the  ovuliferous  scale  (diagrammatic);  6,  megaspore;  7,  pollen- 
chamber;  /,  longitudinal  section  of  a  mature  cone;  6.  ovule;  /,  scale 
from  a  mature  cone;  6,  seed;  ic,  wing  of  seed;  K,  dissection  of  mature  seed; 
//,  hard  seed  coat;  c,  dry  membraneous  remains  of  the  nucellus,  here  folded 
back  to  show  the  endosperm  and  embryo;  e,  embryo;  p,  remains  of 
nucellus;  L,  embryo;  c,  cotyledons;  e,  hypocotyl;  r,  root-end. 

good  reasons  for  considering  that  they  are  not  simple  spor- 
ophylls,  but  are  of  a  more  complex  character. 

Each  scale  comprises  a  bract  and  an  ovuliferous  (ovule- 
bearing)  scale  (Fig.  309).     It  is  probable  (but  not  certain) 


420 


STRUCTURE    AND    LIFE    HISTORIES 


that    the    ovuliferous    scale    represents    two    sporophylls 
fused  together. 

372.  Megasporangia. — As  in  Cycas,  the  megaspor- 
angium  is  an  ovule,  comprising  a  nucellus  (sporangium 
proper),  surrounded  by  an  integument.  At  the  apex  the 
integument    does    not    come    quite    together,    so    that    a 


Fig.  310. — Median  longitudinal  section  of  a  pine  cone, 
scale;  ov,  ovule  at  the  base  of  a  scale. 


ovuliferous 


tiny  opening  or  pore  (the  micropyle)  is  left.  The  micropyle 
leads  to  the  pollen-chamber  below  (Figs.  309  and  311). 
The  megaspores  are  more  backward  in  development  than 
the  microspores,  the  spore-mother-cell  not  appearing  until 
June.  Reduction  by  two  divisions  gives  rise  to  either 
three  or  four  megaspores  in  a  row,  but  only  the  basal  one 
ever  germinates;  the  others  become  disorganized  and 
furnish  nourishment  for  the  one  that  germinates. 


SEED-BEARING    PLANTS  42 1 

373.  Female  Gametophyte. — During  the  first  season  the 
megaspore  (Fig.  311)  enlarges  somewhat,  and  its  nucleus 
divides  several  times,  forming  free  nuclei.  In  this  condi- 
tion it  remains  until  the  next  season,  when  the  formation 
of  the  gametophyte  is  carried  to  completion.  As  the 
gametophyte  develops  it  feeds  on  the  nucellus,  which  is 
entirely  consumed  except  for  a  thin  membrane,   which 


Fig.  311. — White  pine  {Pinus  Slrobus).  At  lef t,  megasporangium  with 
megaspore  in  the  center;  above,  pollen  grains  in  the  micropyle  and  pollen 
chamber.  At  right,  pollen  grains  beginning  to  germinate;  the  cells  of 
the  integument  hav^e  enlarged  and  closed  the  micropyle.  (After  Margaret 
C.  Ferguson.) 

adheres  to  the  surface  of  the  gametophyte  or  endosperm, 
and  a  cap  of  tissue  at  the  tip  of  the  gametophyte  (Figs. 
309,  K,  and  312).^  The  archegonia,  tw^o  to  five  in  number, 
at  the  micropylar  end  of  the  gametophyte,  are  mature  by 
the  last  of  May  or  forepart  of  June,  in  the  northern  states. 
In  Pinus  the  neck  of  the  archegonium  is  very  much  re- 

^  In  the  seeds  of  some  of  the  higher  plants  the  tissue  of  the  nucellus 
becomes  filled  with  nourishment  stored  for  the  use  of  the  developing 
embryo,  during  germination.     It  is  then  called  perisperm. 


422 


STRUCTURE    AND    LIFE    HISTORIES 


duced,  consisting  usually,  in  the  white  pine,  of  only  four 
cells,  all  in  the  same  plane;  the  number  of  these  cells  is 
somewhat  variable.  No  neck-canal  cells  are  formed;  only 
the  egg,  and  the  ventral  canal-cell  (the  sister-cell  of  the 
egg)  which  disorganizes  early  (Fig.  312). 


Fig.  312. — White  pine  {Pinns  Strohus).  \'ertical  section  through  tlie 
upper  part  of  an  ovule,  shortly  before  fertilization,  s.n,  sperm-nuclei; 
st.c,  stalk-cell;  t.n,  tube-nucleus;  arch,  archegonium;  e.n,  egg-nucleus. 
(After  Margaret  C.  Ferguson.) 

374.  Male  Gametophyte. — The  germination  of  the 
microspore  consists  chiefly  of  a  series  of  cell-divisions,  all 
within  the  wall  of  the  microspore.  The  first  three 
divisions  result  in  the  formation  of  four  cells,  namely,  two 
prothallial  cells,  one  mother-cell  of  the  antheridium,  and  a 


SEED-BEARING    PLANTS 


423 


larger  cell,  the  tube-cell,  composing  the  larger  portion  of 
what  is  now  the  mature  male  gametophyte  or  pollen-grain. 
Commonly  one  of  the  two  prothallial  cells  disintegrates, 
so  that  only  one  is  visible  (Fig.  313).  Thus  it  is  seen  that 
the  vegetative  portion  of  the  male  gametophyte  is  reduced 
to  nearly  its  lowest  terms — only  one  or  two  cells  of  no 
known  function;  the  antheridium  is  also  represented  only 
by  its  mother-cell.     At  this  stage  pollination  occurs. 

375.  Distribution  of  Pollen. — In  all  cone-bearing  trees 
pollination  is  accomplished  by  the  wind.     At  about  the 


Fig.  313. — The  white  pine  {Pinus  Strobns).  Sections  through  mature 
pollen  grains;  at  the  left  the  remnants  of  two  prothallial  cells  can  be  seen, 
while  at  the  right  all  signs  of  the  first  cell  have  disappeared.  Pollen  col- 
lected June  9,  1898.     X  about  600.     (After  Margaret  C.  Ferguson.) 

time  the  pollen-grain  is  mature  the  axes  of  the  carpellate 
cones  elongate,  thus  separating  the  scales  from  each 
other  (Fig.  309,  E),  At  the  same  period  the  axes  of  the 
staminate  cones  elongate,  separating  the  anthers  from 
each  other  (Fig.  306).  The  sporangial  walls  now  become 
opened  by  a  longitudinal  slit,  and  the  least  jarring  of  the 
branch  is  sufficient  to  shake  out  the  dry  pollen-grains, 
which  appear  in  countless  millions  as  a  fine  yellow  dust, 
the  ''pollen.'' 

376.  Abundance  of  Pollen. — The  pollen  is  so  abundant 
that  it  forms  a  really  dense  cloud  that  is  easily  seen  in  a 
photograph  of  a  tree  shedding  its  polien  (Fig.  314).     The 


424 


STRUCTURE    AND    LIFE    HISTORIES 


pollen-grains  are,  of  course,  blown  hither  and  thither  with 
every  breeze,  and  millions  of  them  never  reach  a  carpellate 
cone.  The  writer  once  found  an  accumulation  of  pine 
pollen  in  a  desk  drawer  that  had  remained  constantly 
closed  (but  in  the  vicinity  of  a  pine  tree)  during  the 
season    of    pollination.     A    microscopic    examination    of 


Fig.  314. — Shedding  of  pollen  from  a  xoun-  pine  tree.     Note  the  cloud 
of  pollen  at  the  left,  caused  b}^  shaking  the  tree. 


dust  from  ledges,  indoors  and  out,  at  the  pollen  season, 
will  usually  disclose  one  or  more  pollen-grains  of  pines 
and  other  species. 

377.  Pollen  and  Coal-formation. — A  microscopic  ex- 
amination of  muck  from  the  bottom  of  almost  any  in- 
land lake  will  disclose  the  fact  that  it  contains  millions  of 
pollen-grains  of  various  cone-bearing  trees,  and  spores  of 


SEED-BEARING    PLANTS 


425 


Fig,  315. — The  organization  of  shallow  water  accumulations  in  a  lake 
of  the  present  geological  epoch,  showing  remains  of  roots,  leaves,  etc. 
Near  the  center  of  the  figure  may  be  seen  a  cross-section  of  the  triangular 
needle  of  a  white  pine.     (Cf.  Figs.  316  and  317.)     (After  E.  C.  Jeffrey.) 


Fig.  316. — Fine  muck  from  the  bottom  of  a  pond,  as  seen  under  the 
compound  microscope,  showing  the  presence  of  pollen  grains  of  pine, 
spruce,  and  fir.     (Cf.  Figs.  315  and  317.)     (y^.fter  E.  C.  Jeffrey.) 


426  STRUCTURE    AND    LIFE    HISTORIES 

horsetails,  and  other  cryptogams  (Figs.  315  and  316).  It 
is  instructive  in  this  connection  to  recall  recent  careful 
studies  of  the  structure  of  coal  as  seen  in  transparent 
sections  by  the  aid  of  the  microscope.  These  sections, 
prepared  by  Professor  Jeffrey,  reveal  the  remarkable  fact 
that  the  soft,  or  bituminous,  coals  contain  carbonized 
remains  of  innumerable  spores  of  the  plants  which  con- 
stituted the  dominant  vegetation  during  the  geological 


Fig.  317. — Photomicrograph  of  a  thin  section  of  cannel  coal  from 
Kentucky,  formed  under  open-water  conditions,  i.e.,  of  the  muck  at  the 
bottom  of  ancient  lakes  or  lagoons.  The  light,  roundish  bodies  are  spores. 
(Cf.  Figs.  315  and  316.)     (After  E.  C.  Jeffrey.) 

period  (Carboniferous),  when  coal  was  being  formed  (Fig. 
317).  Such  studies  necessitate  a  radical  change  in  our 
earlier  conception  as  to  the  conditions  and  method  of 
coal-formation. 

These  facts  also  illustrate  how  the  study  of  what  might, 
at  first  thought,  seem  insignificant,  impractical,  or  unim- 
portant, and  not  closely  related  to  our  daily  lives,  may, 
at  any  time,  furnish  the  key  to  unlock  the  mystery  of 


SEED-BEARING    PLANTS  427 

5ome    very    important    fundamental    fact    or    scientific 
principle. 

The  formation  of  pollen  in  such  abundance  is  one  of 
the  numerous  instances  of  the  ''factor  of  safety"  in  plant 
organization;  and  the  necessity  for  it  is  recognized  at 
once  when  one  considers  the  small  chance  that  any  given 
pollen-grain  will  reach  the  pollen-chamber  of  an  ovule. 

378.  Pollination. — Some  of  the  pollen-grains,  of  course 
reach  the  carpellate  cones,  which  are  usually  situated 
higher  up  on  the  tree  and  higher  up  on  the  individual 
shoots,  than  are  the  staminate  cones.  This  location  is 
an  advantage,  because  the  light  pollen-grains,  specially 
buoyant  because  of  their  two  air-sacs,  readily  float  up- 
ward. Those  that  reach  the  carpellate  cones,  fall  between 
the  ovuliferous  scales,  and  settle  down  to  the  bases  of 
the  scales.  Some  of  them  get  caught  in  the  sticky  fluid 
that  fills  the  pollen-chamber  at  this  time,  and  as  the  fluid 
dries  up  the  grains  are  drawn  close  down  to  the  tip  of  the 
nucellus  (Fig.  311).  Here,  as  always,  the  deposit  of  pollen 
on  the  surface  where  it  is  to  germinate  is  called  pollina- 
tion. PoUination  in  Pinus  occurs  in  late  May  or  early 
June,  depending  on  the  species,  the  locality,  and  the  nature 
of  the  season. 

379.  Nodding  of  the  Cone. — Soon  after  pollination  the 
stalk  of  the  carpellate  cone,  in  most  species,  changes  its 
relation  to  gravity,  becoming  negatively  geotropic.  One 
side  grows  more  rapidly  than  the  others,  thus  causing  the 
cone  to  nod  and  hang  pendant  (Figs.  307  and  308).  This 
position  it  retains  throughout  the  remainder  of  its  life. 

380.  Germination  of  the  Pollen-grain. — Very  soon  after 
pollination,  the  tube-cell  begins  to  develop  a  pollen- tube, 
which  secretes  an  enzyme  that  dissolves  the  cell-walls  and 


428 


STRUCTURE    AND    LIFE    HISTORIES 


contents  of  the  nucellar  tissue,  thus  facilitating  the 
passage  of  the  dehcate  tube.  The  dissolved  con- 
tents nourish  the  growing  tube,  which  at  first  serves 
as  a  haustorium  to  absorb  the 
nourishment.  Thus  the  male  (as 
well  as  the  female)  gametophyte, 
lives  as  a  parasite  upon  the  tissue 
of  the  sporophyte. 

Soon  after  the  pollen-tube  has 
begun  to  form,  the  tube-nucleus 
moves  down  toward  the  tip, 
where  it  remains,  presiding  over 
the  subsequent  growth  of  the  tube 
(Fig.  318).  At  about  this  time, 
also,  the  antheridial  mother-cell 
divides,  forming  a  wall-cell^  and 
a  generative  cell.  In  this  condi- 
tion the  first  winter  is  passed. 
After  pollination  the  carpellate 
scales,  by  growth,  are  brought 
„     „„  .       .        close  together,  and  secrete  a  very 

Fig.  318.— White   pine         .  .  ^ 

(Pinus  Strobus).  Germi-  sticky,  resmous  substancc,  all  of 
T%f"t2-l7;  which  very  completely  excludes 
g.c,  generative  cell;   /.»,    any  water  from  between  the  scales. 

tube-nucleus.       X    about      „,  .  .  ,       . 

236.    (After  Margaret  c.     The  cone  then  mcreases  greatly  m 

Ferguson.)  gj^^  (^ig.  309). 

381.  Fertilization. — Early  in  the  following  spring  (May- 
June),  the  generative  cell,  after  passing  into  the  pollen- 
tube,  divides  to  form  two  sperm-cells,  and  the  pollen-tube 
continues  its  growth  toward  the  archegonia.  By  this 
time  (June)  the  egg  lies  mature  within,  and  completely 

^  Also  commonly  called  ''stalk-cell." 


SEED-BEARING   PLANTS 


429 


filling,  the  venter  of  the  archegonium.  The  pollen- tube 
passes  between  the  neck-cells  of  the  archegonium,  but 
does  not  ordinarily  enter  the  venter.  The  apex  of  the 
tube  is  ruptured,  probably  by  internal  osmotic  pressure, 
and  its  entire  contents  are  emptied  into  the  cytoplasm 
of  the  egg.  One  of  the  sperm-nuclei  unites  with  the 
egg-nucleus  (June  of  the  second  season),  and  fertilization 
is  accomplished  (Fig.  319). 


Fig.  319.  —White  pine  {Pinus  Sir  oh  us).  Longitudinal  section  through 
an  archegonium  at  the  time  of  fertilization.  Above  the -fusing  nuclei  are 
various  other  elements  emptied  into  the  egg  from  the  pollen-tube.  Col- 
lected June  21,  1898.  X  about  62.  s.g,  starch  grains;  p.r,  prothallium; 
c.p.t,  cytoplasm  from  pollen-tube;  st.c,  stalk-cell;  t.n,  tube-nucleus;  s.n, 
sperm-nucleus;  e.n^  egg-nucleus;  n.s,  nutritive  spheres.  (After  Margaret 
C.  Ferguson.) 

382.  Formation  of  the  Seed. — After  fertilization  the 
oosperm  begins  at  once  to  develop,  giving  rise  to  three 
distinct  structures;  the  pro-embryo,  the  suspensor,  and 
the  embryo-sporophyte.  During  the  early  divisions  of 
the  fertilized  egg  the  male  and  female  chromatins  can  be 
clearly  distinguished  (Fig.  320).  At  the  same  time  the 
adjacent  tissues  of  the  ovule  become  transformed.  A 
portion  of  the  prothallus  or  gametophyte  nourishes  the 
developing  embryo,   but   the  large   bulk  of  it  becomes 


430  STRUCTURE    AND    LIFE   HISTORIES 

Stored  as  endosperm  around  the  embryo.  The  remains 
of  the  nucellus  persist  as  a  thin  membrane  surrounding 
the  endosperm  (as  mentioned  above),  the  integument  of 
the  ovule  develops  into  the  seed-coat,  and  from  the  integu- 
ment there  also  develops  a  long,  thin,  membranous  wing. 
383.  Seed-dispersal. — From  the  above  description  we 
learn  that  it  takes  about  a  year  and  a  half  to  make  a  pine 
seed.     When  the  seeds  are  mature,  the  scales  of  the  car- 


■/^^tOKiVii'^- 


Fig.  320.  —White  pine  {Pinus  Strohus).  Late  prophase  in  the  first 
nuclear  division  of  the  fertilized  egg.  The  nuclear  membrane  has  disap- 
peared, and  the  chromatin  from  both  egg  and  sperm  may  still  be  dis- 
tinguished.    X  about  236.     (After  JMargaret  C.  Ferguson.) 

pellate  cone,  which  have  now  become  large  and  woody, 
spread  apart  (Figs.  308  and  309,  D),  and  thus  permit  the 
loose  seeds  to  fall  out.  By  means  of  the  membranous 
wing,  the  seeds  are  easily  dispersed  by  the  wind. 

384.  Germination  of  the  Seed. — The  seeds  usually  do 
not  germinate  until  the  spring  after  they  are  dispersed, 
or  two  years  after  pollination.  Under  suitable  condi- 
tions of  environment  the  hypocotyl  elongates,  forming  an 
arch,  and  drawing  the  cotyledons  out  of  the  ground, 
while  the  tap-root  develops  from  the  opposite  end.  By 
the  straightening  of  the  arch  the  green  cotyledons  are 
lifted  into  the  air  and  light,  the  hypocotyl  elongates,  the 
root-system  begins  to  develop,  and  thus  the  seedling 
sporophyte  becomes  established  as  an  independent  plant. 


SEED-BKARING    PLANTS 

OUTLINE  OF  LIFE  HISTORY  OF  PINUS 

Tile  pine  tree 
(Sporophyte) 


431 


Carpellate  cone 
(Modified  branch) 

ii 

Ovuliferous  scale 
(Homology  obscure) 

4.4. 

Ovule 
(Megasporangium) 

ii 

Megaspore-mother-cell 

•4'     ^ Reduction 

Megaspore 

I 

Embryo-sac 
(Female  gametophyte) 

i 

Archegonium 
Egg 


TT 

Staminate  cone 
(Modified  branch) 

4-4- 

Stamens 
(Microsporophylls) 

4'  4' 

Anthers 
(Microsporangia) 

Microspore-mother-cell 

^  4. 

Microspore 

4. 

Pollen-grain 
(Male  gametophyte) 

4' 

(Antheridial  cell) 

Sperm 


Oosperm 

4.4. 

Embryo  (In  a  seed^ 
(Zygote) 

4.4. 

Embryo-rests 

4-i 

Germination  of  seed 

4-4- 

Mature  sporophyte  (The  i)ine  tree) 

385.  Comparison  with  Lower  Forms.— It  will  be  spe- 
cially instructive,  at  this  point,  for  the  student  to  make  a 
careful  comparison  of  the  life  history  of  Finns  with  the 
lower  forms,  such  as  Cycas,  Selaginella,  Lycopodium,  EquU 
setum,  and  a  true  fern,  noting  especially  the  relative  impor^ 


432 


STRUCTURE    AND    LIFE   HISTORIES 


tance,  in  the  ascending  scale,  of  gametophyte  and  sporo- 
phyte,  and  the  evidences  which  point  to  a  possible  mode  of 
origin  of  the  vegetative  body  of  the  sporophyte.  A  funda- 
mental question,  still  being  debated  by  botanists,  is 
whether,  in  racial  history,  the  vegetative  (sterile)  tissue 
appeared  first,  and  the  sporogenous  (fertile)  tissue  later, 


Siaqe 


Fig.  321.^ — Diagram  of  the  life-cycle  of  a  pine.     (After  SchaEfner.) 

or  whether  the  fertile  tissue  appeared  first.  The  question 
may  be  put  as  follows:  Is  the  cone  (strobilus)  the  older 
structure  in  the  development  of  the  race,  and  are  the  vege- 
tative parts  to  be  regarded  as  developed  from  it  by  pro- 
gressive sterilization?  It  is  better  that  this  question  be 
merely  stated  here,  as  one  of  the  larger,  fundamental 
problems  of  botany,  and  that  the  pleasure  of  discussing 
it  be  reserved  for  the  teacher  and  students  together,  in 
connection  with  a  later  chapter.     (Cf.,  also,  p.  379.) 


CHAPTER  XXVII 

SEED-BEARING  PLANTS  (Continued) 

LIFE  HISTORY  OF  AN  ANGIOSPERM 

386.  Variations  in  Life  Histories. — In  the  groups  pre- 
viously studied  there  is  a  marked  uniformity  or  similarity 
in  the  life  histories,  making  it  comparatively  simple,  once 
one  has  the  key,  to  interpret  the  structures  involved. 
Even  as  we  pass  from  one  group  to  the  next,  homologies 
are  detected  without  serious  difficulty.  Under  various 
more  or  less  transparent  disguises,  we  have  been  able  to 
trace  such  structures  as  the  sporophyll,  spore-case, 
microspore  and  male  gametophyte,  megaspore  and  female 
gametophyte,  and,  in  the  latter,  the  archegonium,  egg,  and 
embryo.  But  studies  of  the  highest  group  of  plants,  the 
Angiosperms,  soon  lead  us  into  difficulties  not  readily 
overcome.  It  is  not  difficult  to  detect  the  sporophylls, 
spore-cases,  megaspores  and  microspores;  but,  just  as 
among  the  Gymnosperms  the  antheridium  had  dis- 
appeared as  a  distinct,  fully  developed  organ,  so  among 
the  Angiosperms  the  archegonium  has  disappeared,  and 
certain  new  structures  have  made  their  appearance.  Thus 
it  is  not  possible  to  choose  from  the  Angiosperms  any 
actual  plant  whose  life  history,  in  detail,  is  typical  of  the 
entire  group.  For  external  features  any  one  of  several 
plants  might  be  chosen  to  illustrate  the  floral  organs 
commonly  met  with.  The  life  history  of  the  yellow 
28  433 


434 


STRUCTURE   AND    LIFE    HISTORIES 


adder's-tongue,    or    dog's-tooth     violet,    illustrates     the 
essential  points  for  Angiosperms. 

387.  Dog*s-Tooth  Violet. i— The  dog's-tooth  violet  {Ery~ 
thronium  americamim)  belongs  to  the  order  which  includes 


Fig.  322. — Dog's-loolh  violet  {Erylhronium  amcricanum).  Stages  of 
deveopment  from  the  seed.  1-5  show  the  stage  of  development  in  each 
of  live  successive  years.  Full  explanation  in  the  text.  6,  Bulb  showing 
a  surface  bud  (the  sprout  has  been  destroyed).     (After  F.  H.  Blodgett.) 

the  hlies  (Liliales),  and  its  structure  is  quite  typical  of 
that  order  (Fig.  322).  Its  stem  is  a  small,  underground, 
scaly  bulb,  giving  rise  to  numerous  roots.     From  the  upper 

^  The  dog's-tooth  violet  is  really  not  a  violet  at  all,  the  common  name, 
as  frequently,  having  no  regard  to  botanical  relationships.  John  Bur- 
roughs has  suggested  that  "fawn-lily"  would  be  a  much  more  appropriate 
name.  But  common  names  of  plants  and  animals  are,  fortunately,  not 
easily  changed. 


SEED-BEARING   PLANTS  435 

surface  of  the  stem  arise  two  smooth  leaves,  with  a  shiny, 
but  mottled  surface,  and  acute  apex.  The  petioles  sheathe 
the  base  of  a  flower-stalk  {scape),  which  also  arises  from 
the  upper  surface  of  the  bulb.  At  the  tip  of  the  flower- 
stalk  is  the  solitary  flower. 

388.  Blossoming. — Early  in  the  spring  the  flower-stalk 
begins  to  elongate  rapidly  until  it  has  developed  into  a 
long,  slender,  unbranched  stem,  the  scape,  bearing  at  its 
tip  the  flower-bud,  raised  several  inches  above  the  ground, 
and  soon  expanding  into  a  flower.  During  the  forma- 
tion of  the  flower-bud,  in  the  preceding  autumn,  the  outer 
surfaces  of  the  floral  envelopes  grew  more  rapidly  than  the 
inner  surface,  thus  causing  the  formation  of  the  bud.  The 
opening  of  the  bud  is  caused  largely  by  the  more  rapid 
growth  of  the  inner  surfaces  of  the  floral  envelopes. 

389.  Structure  of  the  Flower. — In  Erythronium  (or  any 
liliaceous  plant;  cf.  Fig.  323)  we  may  recognize  all  the  parts 
of  a  complete  flower,  as  follows: 

1.  An  outer  circle  of  three  sepals,  together  constituting 
the  calyx. 

2.  An  inner  circle  of  three  petals,  alternating  with  the 
sepals,  and  together  constituting  the  corolla.  The  sepals 
and  petals  in  Erythronium  look  very  much  alike,  but  each 
petal  has  a  nectary,  or  gland  secreting  nectar,  at  its  base. 
The  calyx  and  corolla  together  constitute  the  perianth. 

3.  Two  inner  circles  of  microsporophylls,  the  stamens, 
three  in  each  circle,  one  opposite  each  sepal,  and  one 
opposite  each  petal.  All  the  stamens,  taken  together 
constitute  the  androecium.* 

*  In  Erythronium  three  of  the  six  stamens  are  frequently  noticeably 
shorter  than  the  other  three,  and  mature  their  pollen  later.  This  is  ex- 
ceptional in  the  Lily  family,  to  which  Erythronium  belongs. 


436  STRUCTURE    AND    LIFE    HISTORIES 

Each  stamen  consists  of  a  slender  stalk,  the  filament, 
bearing  at  its  tip  two  microsporangia  {pollen-sacs) , 
united  to  form  the  anther.  In  the  pollen-sacs  are  numer- 
ous microspores,  which  finally  develop  into  pollen- 
grains. 


Fig.  323. — Wood  lily  {Lilium  philadelphicum). 

A  central  organ,  the  pistil,  composed  of  three  mega- 
sporophylls  (carpels)  united,  and  enclosing  the  mega- 
sporangia  (ovules).  All  the  pistils  taken  together  (one  or 
more)  constitute  the  gynoccimn. 

The  pistil  consists  of  three  distinct  regions  as  follows: 

(a)  The  enlarged  basal  part,  enclosing  the  ovules, 
and  hence  called  the  ovary. 

(b)  A    slender    upward    prolongation    of    the    ovary, 


SEED-BEARING    PLANTS 


437 


called  the  style.  Through  the  center  of  the  style  extends 
a  tiny  canal,  so  that  the  style  is  hollow.  The  walls  of  this 
canal,  in  Erythronium,  are  lined  with  a  glandular  layer  of 
cells  forming  the  conducting  tissue,  which  serves  to  nourish 
the  pollen- tubes  (Fig.  324).  An  expansion  of  this  tissue  is 
exposed  at  the  tip  of  the  style,  forming  the  stigma,  or  surface 
for  the  reception  of  the  pollen-grains  in  polHnation.  The 
conducting  tissue  extends  continuously  from  the  stigma 
down  through  the  style  to  the  placenta,  or  point  of  attach- 
ment of  the  ovules. 


Fig.  324. — ErytJironium  americannm.  i,  longitudinal  section  of  hollow 
style,  showing  glandular  cells  of  the  conducting  tissue  lining  the  canal; 
2,  microspore;  3,  young  pollen  grain  (male  gametophyte),  showing  gen- 
erative nucleus  and  tube  nucleus;  4,  pollen-grain  germinating,  i  and  4 
are  from  E.  albidum.     (After  J.  H.  Schaflfner.) 


In  many  plants  the  style  is  not  hollow,  and  the  con- 
ducting tissue  completely  fills  the  center.  The  stig- 
matic  surface  secretes  a  sticky  substance  by  which  the 
pollen-grains  are  held  fast.  In  some  species  of  plants 
this  surface  is  covered  with  tiny  hairs,  by  which  the 
pollen-grains  are  held  until  they  germinate. 

390.  Pollination. — The  flower  of  Erythronium  stands 
out  in  sharp  contrast  to  that  of  the  gymnosperms,  in  two 
respects,  namely,  the  occurrence  of  both  stamens  and 
carpels  in  the  same  flower,  and  the  possession  of  a  con- 
spicuous,   colored    perianth.     The    significance    of    the 


438  STRUCTURE    AND    LIFE    HISTORIES 

perianth  can  be  understood  only  in  connection  with 
pollination.  It  will  be  recalled  that  in  the  gymnosperms 
the  pollen  is  transferred  by  wind,  but  in  Erythronium 
this  transfer  is  accomplished  by  means  of  insects.  The 
perianth,  conspicuous  to  us  by  its  petals,  appears  to  at- 
tract certain  insects.  Whether  this  is  accomplished  by 
color,  or  by  odor,  or  by  some  other  means  not  clearly 
demonstrated,  is  not  absolutely  known.  It  is  generally 
believed  to  be  by  color,  but  certain  experiments  seem  to 
disprove  this  theory.  Be  that  as  it  may,  we  know  that 
the  development  of  a  conspicuous  perianth  appeared  in 
the  same  geological  age  (Cretaceous)  as  did  the  more 
highly  developed,  winged  insects,  such  as  the  butterflies 
and  moths.  In  fact  the  insects  probably  appeared  some- 
what earlier  than  the  *^ flowers." 

Attracted  to  the  flowers  by  whatever  means,  the  insect 
finds,  at  the  bases  of  the  petals,  nectar  secreted  by  glands. 
While  feeding  on  the  nectar  the  back  of  the  insect  becomes 
dusted  over  with  pollen  from  the  anthers.  When  he 
flies  to  another  flower  some  of  this  pollen  is  rubbed  off 
on  to  its  stigma,  thus  accomplishing  pollination. 

391.  The  Male  Gametophyte. — The  young  pollen-grain 
has  already  been  recognized  as  a  microspore.  In  some 
species  it  develops  into  a  male  gametophyte  before  pollina- 
tion, in  other  cases  not  until  afterward.  In  either  case 
the  gametophyte  is  very  greatly  reduced.  The  mature 
pollen-grain  of  the  milkweed,  for  example,  is  a  mature 
gametophyte,  having  the  sperm-cells  formed  at  about  the 
time  the  flower-buds  open.  In  Erythronium  the  genera- 
tive cell,  formed  between  December  i  and  April  i,  does 
not  divide  to  form  the  sperm-cells  until  after  pollination, 
and  after  the  pollen-tube  has  begun  to  form  (Fig.  324). 


SEED-BEARING    PLANTS 


439 


392.  The  Female  Gametophyte. — Megaspores  are 
probably  not  formed  in  tetrads  by  two  divisions  of  a 
megaspore-mother-cell,  but  the  ancestral-cell  of  the  female 


Fig.  325. — Dog's-tooth  violet  {Erylhronium).  i,  embryo-sac,  binucle- 
ate  stage,  the  two  nuclei  dividing;  2,  four  nucleate  stage;  3,  mature  embryo- 
sac  {es))  the  pollen-tube  {p.t)  has  reached  the  egg-apparatus  and  fertiliza- 
tion has  just  taken  place;  the  male  and  female  nuclei  are  both  visible  in 
the  fertilized  egg.  The  synergids  are  not  shown;  4,  polyembryony;  four 
embryos  have  developed  from  the  embryogenous  tissue,  one  embryo  at 
the  left  of  a  not  shaded;  5,  longitudinal  section  of  an  embryo  {em);  s, 
suspensor;  i,  2,  and  5,  E.  albidum  (after  Schaffner);  3  and  4,  E.  americamim 
(after  Jeffrey).     Only  one  of  the  multiple  embryos  persists. 

gametophyte  {archesporial  cell)  functions  directly  as  a 
megaspore,  without  division.  In  very  early  spring  the 
megaspore   begins  to  germinate,  increasing  in  size,  and 


440 


STRUCTURE    AND    LIFE    HISTORIES 


its  nucleus  undergoing  divisions,  forming,  in  succession, 
a  two-celled,  four-celled,  and  eight-celled  embryo-sac 
(Figs.  325  and  326).  Three  of  these  cells  pass  to  one 
end  of  the  sac,  opposite  the  micropyle,  and  are  known 


l>0   St  is 


Fig.  326. — At  the  left,  diagram  of  the  anatomy  of  an  angiospermous 
flower  shortly  after  pollination;  anlh.,  anther;  fd.,  filament;  st.,  stamen; 
slig.,  stigma;  p.g.,  pollen  grains  germinating;  sly.,  style;  pL,  pollen  tube; 
o.'co.,  ovary  wall;  o.,  ovule,  containing  embryo-sac;  pet.,  petal;  sep.,  sepal. 
1-8,  Stages  in  the  development  of  the  female  gametophyte  (embryo-sac); 
meg.sp.,  megaspore-mother-cell;  i.i.,  inner  integument;  o.i.,  outer  integu- 
ment; fun.,  funiculus;  dial.,  chalaza;  mi.,  nucellus  (megasporangium); 
emb.,  embryo-sac.     All  diagrammatic. 

as  the  antipodal  cells,  or  antipodals,  while  two  of  them  meet 
in  the  center  and  fuse,  forming  the  endosperm-nucleus. 
The  remaining  three  pass  to  the  end  near  the  micropyle, 
where  one  of  them  becomes  organized  as  the  egg-cell;  the 
others  are  called  synergids  (helpers)  (Cf.  Fig.  326). 


SEED-BEARING    PLANTS 


441 


393.  Fertilization  and  Formation  of  Embryo. — After 
pollination  the  pollen-grain,  stimulated  by  the  sticky 
substance  secreted  by  the  stigma,  begins  to  develop  a 
pollen-tube,  which  passes  down  the  canal  that  extends 
through  the  style  from  stigma  to  placenta,  nourished  by 
the  specialized  cells  that  line  the  inner  surface  of  the 


^k 

i  M 

1 

0  H^^ 

w>^ 

>  1^^^ 

f)~''! 

\X» 

W  "' 

^* 

^ 

f  1 

^ 

^  1 

3 

,  V, 

Fig.  327. — Lilium  Martagon.  Longitudinal  section  of  the  stigma  and 
upper  part  of  the  style.  The  pollen-grains,  caught  on  the  papillae  of  the 
stigma,  have  germinated,  and  the  pollen-tubes  are  growing  down  along 
the  walls  of  the  style-canal,  nourished  by  the  specialized  cells  that  line  it. 
(After  Dodel-Port.) 


canal  (Cf.  Figs.  324  and  327).  When  the  tip  of  the  tube 
has  passed  through  the  micropyle  and  into  the  embryo-sac, 
the  sperm-cells,  formed  during  the  growth  of  the  tube,  pass 
out  into  the  protoplasm  of  the  embryo-sac,  and  one  of 
them  fuses  with  the  egg-cell,  thereby  accomplishing  fer- 
tilization (Fig.  328.  Cf.  Fig.  325).  In  some  cases  the 
embryonic  tissue  that  arises  from  the  fertilized  tgg  gives 


442 


STRUCTURE    AND    LIFE    HISTORIES 


rise  to  as  many  as  four  embryos  (Fig.  325),  but  usually 
only  one  of  them  develops. 

394.  Formation  of  the  Seed. — While  the  fertilized  egg 
is   developing  into   the   embryo,    the   endosperm-nucleus 


Fig.  328. — Lilium  canadense.  Embryo-sac  at  the  time  of  fertilization; 
a\  a^,  antipodal  cells;  dn.,  endosperm-nucleus;  pi.,  remains  of  pollen-tube; 
e.n.,  egg-nucleus;  s.n.^,  sperm-nucleus,  fusing  with  the  egg-nucleus;  s.n.^, 
second  sperm-nucleus,  which  may  later  fuse  with  the  endosperm-nucleus, 
thereby  accomplishing  double  fertilization.  (Redrawn  from  camera 
lucida  drawing  by  O.  E.  White.) 

is  undergoing  successive,  rapid  divisions,  which  finally 
result  in  the  formation  of  an  abundance  of  starchy  endo- 
sperm, surrounding  the  embryo,  and  serving  to  nourish  it 


SEED-BEARING    PLANTS  443 

when  the  seed  germinates.^  At  the  same  tmie  the  integu- 
ments of  the  ovule  develop  into  a  hard,  horny  seed-coat. 
This  kind  of  seed-coat  is  characteristic  of  the  Liliaceae. 

395.  Germination  of  the  Seed. — The  seeds  are  mature 
by  about  June,  and  lie  on  the  ground  dormant  until  the 
following  April,  when  they  germinate.  Both  ends  of  the 
embryo  elongate,  absorbing  all  the  endosperm  for  nourish- 
ment. By  about  the  time  that  older  plants  are  blossom- 
ing, the  young  seedling  has  reached  the  stage  shown  in 
Fig.  322,2.  At  one  side,  near  the  end  of  the  hypocotyl, 
there  develops  a  root,  and  the  tip  becomes  enlarged  into 
a  bulb  by  the  storage  of  starch,  manufactured  by  the 
green,  cylindrical  seed-leaf.  Within  this  bulb  the  first 
bud  {plumule)  develops,  the  seed-leaf  withers,  and  the 
young  seedling  remains  in  this  condition  during  the 
following  winter. 

396.  Formation  of  Flower  Bulb. — In  the  spring  of  the 
second  year  several  runners  develop  from  the  first-formed 
or  plumule-bulb,  and  at  their  tips  bulbs  also  form,  called 
runner-bulbs.  From  each  of  the  runner-bulbs,  three  more 
runners,  with  bulbs,  are  produced,  and  one  of  these  bulbs, 
under  favorable  conditions,  produces  a  flowering  plant. 
It  takes  at  least  four  years  to  produce  a  bulb  that  will 
develop  a  flowering  plant. 

''The  following  table  illustrates  the  number  of  plants 
of  different  ages  during  each  of  live  years,  supposing  that 
five  seeds  from  each  fruit  ripen  and  survive  the  cycle,  and 
provided  that  all  fourth  year  bulbs  produce  flowers."^ 

^  It  will  6e  instructive  for  the  class  to  discuss  the  origin  and  mode  of 
formation  of  the  starch  in  the  endosperm. 

2  Quotation  and  table  from  Frederick  H.  Blodgett,  Bull.  Torrey  Club 
27:307-308.     1900. 


444 


STRUCTURE    AND    LIFE    HISTORIES 


ist  year 

2d  year 

3d  year 

4th  year 

5th  year 

5  seeds 

5  seeds 

5  seeds 

5  seeds 

5  seeds 

5  plumule- 

5  plumule- 

5  plumule- 

5  plumule- 

bulbs 

bulbs 

bulbs 

bulbs 

5  yearlings 

5  yearlings 
15  two  years 
old 

5  yearlings 
15  two  years 

old 
45  flowers 

397.  Annual  Bulbs. — At  the  base  of  an  old  flowering 
bulb,  and  in  the  axil  of  one  of  the  bud-scales,  there  de- 


FiG.  329. — Diagram  of  life-cycle  of  an  angiosperm  (Alisma  Planlago- 
aquatica).  9,  female  gametophyte  (embryo-sac);  8a  and  9a,  male  gameto- 
phyte  (pollen-grain).     (After  J.  H.  Schaffner.) 

velops  each  year  an  annual  bulb,  which  attains  full  size 
about  April,  and  begins  in  May  to  form  the  bud  for  the 
leaves  and  flowers  of  next  spring. 


SEED-BEARING    PLANTS  445 

If  one  digs  up  a  bulb  in  the  fall,  he  will  find  the  flower 
perfectly  formed,  and  ready  to  be  raised  above  the  ground 
the  following  spring.  All  the  parts  of  the  flower,  at  this 
period,  are  white,  on  account  of  having  been  formed  in 
entire  darkness;  and  they  are  also  quite  brittle. 

When  the  buds  resume  their  growth  the  second  spring 
they  push  up  through  the  ground  quickly,  and  with  con- 
siderable force.  The  pointed  end  of  the  sprout  is  covered 
by  a  mass  of  hard  tissue,  which  protects  the  more  delicate 
cells  below  from  injury.  The  well-protected  tip,  and  the 
growth-force,  enable  the  sprout  to  pierce  even  small 
twigs.  This  has  given  rise  to  the  striking  name  of  'Vege- 
table awl." 

As  soon  as  the  sprout  is  well  above  the  surface  of  the 
ground  the  flower  bud  becomes  free  from  the  parts  that 
enclose  it,  and  expands  into  the  nodding  blossom;  pollina- 
tion is  accomplished,  and  the  life-cycle  begins  again. 

The  life  cycle  of  another  angiosperm,  the  water-plantain 
{Alisma  Plantago-aquatica) ,  is  indicated  in  Fig.  329. 


CHAPTER  XXVIII 

SEED -BEARING  PLANTS  (Continued) 

ANGIOSPERMS 

398.  Essentials  of  a  Flower. — Reduced  to  its  lowest 

terms,   a  flower  is  a  branch  bearing  sporophylls.     The 


Fig.  330. — Inflorescences  of  Job's  tears  {Coix  lacrima-Jobi),  one  of  the 

Gramineae. 

latter  may  be  microsporophylls  only,  as  in  the  staminate 
cone  of  Pifiiis  (Fig.   306),  which  is  thus  seen  to  be,  in 

446 


SEED-BEARING    PLANTS  447 

reality,  a  flower;  or  they  may  be  megasporophylls.  In 
the  latter  case  they  may  occur  on  the  main  stem,  as  in 
Cycas  (Fig.  291),  or  grouped  on  a  specialized  branch, 
forming  a  cone,  as  in  Macrozamia  (Fig.  289).^ 

399.  Perfect  and  Imperfect  Flowers. — A  flower  having 
stamens  but  no  carpels,   or  carpels  but  no  stamens  i? 


Fig.  331. — Flowers  of  a  tuberous  begonia;  staminate  above;  pistillate 
below;  one  of  the  latter  with  the  perianth  removed  to  show  the  ovary  and 
stigmas.     (Photo  by  Elsie  M.  Kittredge.) 

unable,  by  itself,  to  produce  seed,  and  is  hence  called  an 
imperfect  flower  (Figs.  :^2)^-2)?>s) -  A  species  in  which  the 
imperfect  flowers  occur  on  separate  plants  is  dioecious. 

^  Whether  the  carpellate  cone  of  Finns  is  a  flower  or  a  cluster  of  flowers 
(inflorescence),  has  long  been  debated.  There  is  strong  evidence  for  con- 
sidering it  a  cluster  of  flowers,  since  the  individual  scales  are  probably  not 
simple  sporophylls.     (Cf.,  p.  419.) 


448 


STRUCTURE    AND    LIFE    HISTORIES 


Such  is  the  case  in  the  willow,  hop,  ailanthus  and,  of  course, 
the  cycads.  When  the  staminate  and  pistillate  flowers 
occur  on  the  same  plant,  either  on  the  same  branch  or 
axis,  as  in  cat-tail,  "Job's  tears,"  begonia,  et  cetera  (Figs. 


Fig.  332. — Inflorescences  of  the  birch  {Betula  sp.).     Below,  the  staminiiU' 
flowers  in  large,  pendant  catkins;  above,  the  pistillate  catkins,  erect. 

330-332,  375)  or  on  separate  branches,  as  in  Indian 
corn,  arrow-leaf,  and  others  (Fig.  333),  the  species  is 
monoecious. 

Since  stamens  and  pistils  are  necessary  to  the  formation 
of  seeds  they  are  called  the  essential  organs  of  the  flower. 
A  flower  like  the  tulip,  rose,  water-arum,  or  buttercup 
(Fig.  345),  having  both  kinds  of  essential  organs,  is  a 
perfect  flower. 


SEED-BEARING    PLANTS  449 

400.  Complete  Flowers. — In   the  majority   of  Angio- 
sperms  the  flower,  in  addition  to  the  essential  organs, 


Fig.  333. — Arrow-leaf  (Sagiltaria).  At  the  left,  branches  with  staminate 
flowers  only;  in  the  middle,  branches  with  pistillate  flowers  only;  at  the 
right,  pistillate  branch,  bearing  fruit. 


possesses  a  calyx  or  a  corolla,  or  both.     Flowers  possessing 
both  kinds  of  floral  envelopes  and  both  kinds  of  essential 
organs  are  called  complete. 
29 


450 


STRUCTURE    AND    LIFE    HISTORIES 


Fig.  334. — Fruit  of  the  tomato  {Lycopersiciim  esciilentum) .  A  berry, 
showing  seeds  (ripened  ovules)  attached  to  the  placentas,  and  inclosed  in 
the  tissue  of  the  ripened  ovary  {i.e.,  angiospermous). 


Fig.  335. — Rosa  rugosa,  at  left;  CratcBgus  punctata,  at  right.  The 
fruit  is  composed  of  the  ripened  ovary,  reinforced  by  the  enlarged 
receptacle. 


SEED-BEARING   PLANTS  45 1 

401.  Essentials  of  a  Fruit. — In  the  Gymnosperms  we 
found  the  seeds  unprotected  on  the  surface  of  the  mega- 
sporophyll  or  carpel;  but  in  the  Angiosperms  the  ovules 
are  produced  in  a  closed  ovary  composed  of  one  or  more 
carpels  (Fig.  334).  As  the  ovules  ripen  into  seeds  the 
carpels  and  surrounding  parts  ripen  into  the  fruit.  In 
some  cases  the  fruit  consists  only  of  the  ripened  ovary 
(Fig.  334)  while  in  other  cases  it  may  comprise  the  enlarged 
calyx  and  receptacle  also  (Fig.  335). 

402.  Immediate  Effect  of  Pollen.— The  effect  of  the 
germinating  pollen  in  stimulating  the  growth  of  the  ovary 
and  adjacent  tissues  is  a  very  interesting  phenomenon. 
A  portion  of  the  edible  part  of  the  fruit  of  apples  is  calyx, 
which  has  developed  into  fleshy  tissue  as  a  result  of 
the  stimulus  of  the  pollen;  in  the  case  of  pears  the  re- 
ceptacle and  end  of  the  peduncle  become  fleshy  and  form 
a  part  of  the  fruit;  most  of  the  strawberry  fruit  is  the 
common  receptacle  of  the  small  flowers,  stimulated 
to  a  fleshy  development  by  the  growth  of  the  pollen 
on  the  stigmas;  in  the  watermelon,  orange,  tomato,  and 
many  other  plants,  it  is  the  ovary  alone  that  is  thus 
stimulated. 

The  immediate  effect  of  pollen  is  often  greatly  increased 
by  cross-pollination.  This  is  strikingly  shown  in  the 
blueberries  (Vaccinium),  as  shown  in  Fig.  336.  The 
two  twigs  "grew  in  equally  good  situations  on  the  same 
bush,  contained  the  same  number  of  flowers,  all  pol- 
linated by  hand  with  equal  care,  and  the  fruits  were  pol- 
linated on  the  same  day.  The  only  difference  in  treat- 
ment was  that  the  pollen  used  on  the  left-hand  twig  came 
from  other  flowers  on  the  same  bush,  while  the  pollen  for 
the  right-hand  twig  was  taken  from  another  bush." 


452 


STRUCTURE   AND   LIFE   HISTORIES 


I 

J 

^ 

r 

J^ 

■1 

^^^i^^'^ii^^ 

'4 

r  ^\ 

i 

r. 

? 

-^^    ^ J               ^l^v. 

1 

1 

\ 

1    ' 

Fig.  336. — Effect  of  self-pollination  in  the  blueberry  {Vacciniiim  corym- 
bostim),  as  compared  with  cross-pollination.  These  two  twigs,  both 
natural  size,  were  in  equally  good  situations  on  the  same  bush,  contained 
the  same  number  of  flowers,  all  pollinated  by  hand  at  the  same  time  with 
equal  care,  and  the  fruits  were  photographed  on  the  same  day.  The  only 
difference  in  treatment  was  that  the  pollen  used  on  the  left-hand  twig 
came  from  other  flowers  on  the  same  bush,  while  the  pollen  for  the  right- 
hand  twig  was  taken  from  another  bush.  The  cross-pollinated  flowers 
produced  a  full  cluster  of  handsome  fruit.  The  self-pollinated  flowers 
produced  no  ripe  fruit,  all  the  fruit  that  set  remaining  small  and  green  and 
later  dropping  off,  until  at  the  time  the  photograph  was  taken  only  two 
such  imperfect  fruits  remained.  A  plantation  made  up  wholly  from  cut- 
tings from  a  single  bush  would  produce  little  or  no  fruit.  At  least  two 
original  propagation  stocks  are  necessary.  (After  Coville.  Courtesy  of 
the  U.  S.  Dept.  Agric.) 


SEED-BEARING   PLANTS  4^3 

403.  Essentials  of  a  Seed.— In  the  cycads  and  pines 
we  have  seen  accomplished  the  first  step  necessary  for  the 
production  of  a  true  seed,  namely,  the  retention  within 
the  sporangium  of  the  megaspore  and  the  female  gameto- 
phyte  to  which  it  gives  rise.  The  final  step  was  the  forma- 
tion of  an  embryo,  which  usually  rests  before  proceeding 
to  develop  into  an  adult  sporophyte.  With  few  excep- 
tions, the  distinctive  feature  of  a  seed  is  a  resting  embryo. 
The  embryo  may  or  may  not  be  surrounded  by  nourish- 


FiG.  337— John  Ray  (1628-1 705).     An  early  and  noted  English  botanist. 
First  to  distinguish  monocotyledons  from  dicotyledons. 

ment  stored  in  the  form  of  endosperm.  In  the  absence 
of  endosperm,  as  for  example  in  the  bean  seed,  the  nour- 
ishment is  stored  in  the  cells  of  the  embryo  itself,  having 
been  absorbed  while  the  embryo  was  forming.  Enclosing 
the  other  parts  of  a  seed  is  the  seed-coat  which  may  be 
derived  from  one  integument  (as  in  Pinus) ,  or  from  two 
integuments  organically  united.  These  features  are  illus- 
trated in  Fig.  83. 

404.  Monocotyledons   and   Dicotyledons. — Except   in 
rare  cases,  all  plant-embryos  possess  either  one  or  more 


454 


STRUCTURE    AND    LIFE    HISTORIES 


''seed-leaves,"  or  cotyledons,  and  on  this  basis  Ray 
(1628-1705),  the  noted  English  botanist,  divided  his  two 
major  groups,  flowering  herbs  (herbcE  perfectce)  and  trees, 


Fig.  338. — Morphology  of  typical  monocotyledonous  plant.  A^  leaf, 
parallel- veined;  B,  portion  of  stem,  showing  irregular  distribution  of  vas- 
cular bundles;  C,  ground  plan  of  flower  (the  parts  in  3's);  D,  top  view  of 
flower;  E,  seed,  showing  monocotyledonous  embryo. 


into    two    sub-groups    monocotyledons    and    dicotyledons} 

These  two  groups  are  distinguished  by  other  characters 

^  Plants  like  Pinus  having  more  than  two  cotyledons  are  polycotyledons. 


SEED-BEARING   PLANTS 


455 


which  are  quite  constantly  associated  with  the  possession 
of  one  or  two  cotyledons.  Thus,  in  monocotyledons  the 
leaves  are,  with  rare  exceptions,  parallel-veined,  and  the 
growth  of  the  stem  is  endogenous;  while  in  dicotyledons  the 


Fig.  339. — Morphology  of  a  typical  dicotyledonous  plant.  A,  leaf, 
pinnately-netted  veined;  B,  portion  of  stem,  showing  concentric  layers  of 
wood;  C,  ground-plan  of  flower  (the  parts  in  s's);  D,  perspective  of  flower; 
E,  longitudinal  section  of  seed,  showing  dicotyledonous  embryo. 


leaves  are  usually  net- veined,  and  the  stem  exogenous.  In 
monocotyledons,  also,  the  parts  of  the  flower  usually  occur 
in  threes  (as  in  Erythronum) ,  or  in  sixes,  never  in  fives,  while 


456  STRUCTURE    AND    LIFE    HISTORIES 

in  dicotyledons  the  parts  are  typically  in  Jours  or  jives. 
These  characters  are  illustrated  diagrammatically  in  Figs. 
338  and  339. 

405.  Groups  of  Dicotyledons. — There  are  two  main 
groups  of  dicotyledons,  based  on  the  fusion  or  non-fusion 
of  the  parts  of  the  calyx  or  corolla,  as  follows: 

{A     h'  hi  1  [  ^P^^^^® 

\  Polypetalae 
Metachlamydeae        Sympetalae  (Gamopetalae) 

The  distinction  between  Archichlamydeae  and  Sym- 
petalas  is  not  absolute,  since  each  group  contains  plants 
having  some  features  characteristic  of  the  other.  The 
Apetalas,  as  the  name  suggests,  are  without  corolla  (in 
some  cases  without  either  calyx  or  corolla) ;  the  Poly- 
petalae have  sepals  and  petals  (one  or  both)  entirely 
distinct;  while  the  Sympetalae  have  the  sepals  and  the 
petals  wholly  or  partly  united  so  as  to  form  a  tubular  calyx 
or  corolla.  In  the  light  of  our  preceding  study  of  the 
fruiting  branch  or  '^flower"  of  Gymnosperms,  it  will  be 
readily  understood  that  flowers  of  simple  structure  are 
presumably  more  primitive  than  those  of  more  complex 
structure.  The  simplest  flower  we  can  imagine  is  an 
apetalous  staminate  flower  of  one  stamen ;  or  an  apetalous 
pistillate  flower  of  one  simple  pistil.  Polype talous  flowers 
are  more  highly  organized  than  apetalous,  and  may 
therefore  be  less  primitive;  sympetalous  flowers  are 
more  complex  or  more  highly  organized  and  are  therefore 
less  primitive  than  either  Apetalae  or  Polypetalae. 

The  following  examples  will  serve  to  illustrate  1 5  of  the 
more  common  or  more  familiar  families  of  Dicotyledons, 
Dut  of  a  total  of  over  250, 


SEED-BEARING    PLANTS 

ARCHICHLAMYDEJE 

Apetal^ 


457 


406.  Willow  Family  (Salicaceae). — This  family  com- 
prises both  the  willows  (Salix)  and  the  poplars  {Populus). 
There  are  about  20  species  of  poplar  in  North  America. 
The  willows,  in  North  America,  comprise  over  30  species, 


«.  ! 

W^ 

^■■W 

'^^^^''■^M 

'«5p/  ■ 

^^flf-    ^^-  '-^ssm-            li 

1^^ 

Fig.  340. — Inflorescence  of  a  willow  (Salix  discolor).     At  the  left  pistillate, 
at  the  right  staminate  catkins.     (Photo  by  Elsie  M.  Kittredge.) 

trees  and  shrubs,  and  are  specially  common  in  moist 
situations.  Several  species  are  cultivated  to  furnish 
twigs  for  basket-making.  The  weeping  willow  {Salix 
bahylonica),  an  object  of  peculiar  beauty  along  streams 
and  lakeshores,  owes  its  pendulous  or  "weeping"  character 


458  STRUCTURE    AND    LIFE    HISTORIES 

to  its  failure  to  develop  sufficient  mechanical  tissue  (wood) 
in  its  smaller  branches  to  hold  them  erect. 

All  willows  are  dioecious.  The  imperfect,  apetalous 
flowers  occur  crowded  together  on  scaly  spikes  called 
catkins  (Fig.  340).  Each  scale  bears  one  flower  in  its  axil. 
The  staminate  flowers  consist  usually  of  two  (sometimes 


Fig.  341. — Willow  (Salix  exigua  Nutt.)  Leafy  branch,  bearing  two 
pistillate  catkins.  Staminate  flower  above,  at  the  left;  pistillate  flower 
below,  at  the  right.     (After  Britton  and  Brown.) 

three  to  ten)  stamens  (Fig.  341).  In  some  species  the 
stamens  are  united.  When  the  flower  buds  open,  early 
in  spring,  the  numerous  hairs  on  the  scales  or  filaments 
(one  or  both)  give  the  soft,  fur-like  appearance,  which 
suggested  the  name  ''pussy-willow."  Though  a  perianth 
is  wanting  pollination  is  accomplished  by  insects. 


SEED-BEARING    PLANTS 


459 


407.  Lizard's-tail  Family  (Saururaceae). — The  lizard's 
tail  {Saururus  cernuus) ,  typical  of  this  family,  has  apetalous 
but  perfect  flowers,  borne  sessile  on  a  moderately  long, 
common  axis,  called  a  spike  (Figs.  342  and  343).  There 
are  three  to  five  more  or  less  united  ovaries.  The  plant 
is  commonly  found  in  swamps  and  marshes. 


Fig.  342. — Lizard's-tail  {Saururus  cernuus). 

POLYPETAL^ 

408.  Crowfoot  Family  (Ranunculaceae). — The  Latin 
name  Ranunculus  (little  frog)  was  applied  to  the  butter- 
cups by  the  ancient  Roman  naturalist  Pliny,  because  they 
are  common  in  wet  places  where  frogs  abound.  The  name 
of  the  family  is  derived  from  this  generic  name,  but  the 
family  comprises  about  35  genera,  28  of  which  occur  in 


460  STRUCTURE   AND    LIFE    HISTORIES 

Eastern  North  America.  The  yellow  water-crowfoot 
{Ranunculus  delphinifolius)  is  one  of  the  commoner  of  the 
40  to  50  North  American  species.     It  is  found  at  the  bor- 


FiG.  343. — Lizard's-tail  {Saururus  cernmis).     Inflorescence,  about 
natural  size. 

ders  of  quiet  water,  and  the  submerged  leaves  are  iiliformly 
dissected,  in  marked  contrast  to  those  borne  in  the  air. 


SEED-BEARING    PLANTS  46 1 

The  bright  shining,  yellow  petals  vary  in  number  from 
five  to  eight,  and  are  much  longer  than  the  sepals;  each 
has  a  little  scale  at  its  base  concealing  a  nectar-gland.  The 
simple  pistils  {carpels)  are  grouped  in  a  round  head, 
surrounded  by  the  numerous  stamens  (Figs.  344  and  345). 


Fig.  344. — Plant  of  a  buttercup  {Ranunculus  sp.).     (Photo  by  Elsie  M. 

Kittredge. ) 

409.  Spiral  and  Cyclic  Arrangement. — It  will  be  re- 
called that  in  the  lower  type  of  flower,  characteristic  of 
the  Gymnosperms,  the  sporophylls  are  arranged  in  spirals 
on  the  flower  axis.  A  study  of  the  flower  in  Angiosperms 
discloses  a  tendency  for  the  flower  parts  to  occur  in 
circles;  the  higher  the  plant  in  the  system  of  classification 
the  more  completely  is  the  cyclic  arrangement  realized. 
In  the  Crowfoot  family  there  are  some  species,  especially 


462 


STRUCTURE    AND    LIFE    HISTORIES 


of  Ranunculus,  in  which  the  sepals  and  petals  are  in  circles, 
while  the  stamens  and  pistils  are  in  spirals — a  more 
primitive  feature. 

410.  Petalody  of  Bracts. — It  is  also  common  in  certain 
species  of  this  family  (notably  in  the  genera  Trollius  and 


Fig.  345. — Flower  of  a  buttercup  {Ranunculus  sp.);  a,  b,  normal,  show- 
ing 5  petals;  c,  d,  petalody  of  stamens;  e,  petal  with  nectary  at  its  base; 
f-h,  ripened  ovaries. 

Anemone)  for  the  green  foliage-like  bracts  below  the 
perianth  to  assume  the  characters  of  sepals,  and  even  of 
petals,  so  that  frequently  one  can  hardly  say  whether  a 
given  segment  of  the  perianth  is  a  true  petal,  or  a  trans- 
formed bract.  By  the  petalody  of  the  bracts  the  flower 
appears  to  be  ''double." 

411.  Coalescence  of  Petals. — It  frequently  occurs  in 
flowers  of  Ranunculaceae  (and  in  other  normally  poly- 
petalous  families  also)  that  the  initial  stages  or  primordia 


SEED-BEARING    PLANTS 


463 


of  two  or  more  petals  become  wholly  or  partly  fused  or 
coalesced,  thus  reducing  the  number  of  separate  members 
of  the  corolla  (Fig.  346).  Sometimes  coalescence  and 
petalody  of  stamens  will  occur  in  the  same  specimen,  so 
that  a  flower  that  would  normally  have  five  petals  may 
have  six  or  eight,  or  more,  some  of  which  have  coalesced, 
as  is  indicated  by  the  two  or  more  points  at  the  tip. 


Fig.  346. — Rue  anemone  {Ane^nonclla  thalictroides) .  i,  normal  flower 
with  5  petals;  3,  petalody  of  stamens;  4,  coalescence  of  petals  (c^);  2,  coales- 
cence (c),  and  petalody  of  stamens.  At  2,  s  is  shown  a  stamen  partially 
transformed  into  a  petal,  but  with  a  portion  of  the  anther  still  remaining. 


412.  Mustard  Family  (Cruciferae). — The  flowers  of 
the  mustard  family  are  mostly  characterized  by  having 
the  four  narrow  petals  opening  out  at  an  angle  of  90° 
from  each  other,  forming  a  Greek  cross  (Fig.  347),  whence 
the  family  name,  Cruciferae.  This  character  of  the  corolla 
also  appears  in  rare  instances  in  other  famihes  {e.g.^  some 
Rubiaceae),  whose  corolla  is  then  said  to  be  ''cruciferous." 
The  fruit,  a  silique,  is  also  one  of  the  ear-marks  of  the 


464 


STRUCTURE    AND    LIFE    HISTORIES 


family.  The  mustard  family  contains  many  valuable 
economic  plants,  such  as  the  white  and  black  mustard, 
radish,  cabbage,  turnip,  kohlrabi,  and  brussels  sprouts. 

413.  Rose  Family  (Rosaceae). — One  of  the  common 
wild  roses  (Rosa  Carolina)  illustrates  a  type  of  flower 
structure  more  advanced  in  several  ways  than  that  of 
Rammcuhts.     The    flowers,    with    rare    exceptions,    have 


Fig.  347. — Black  mustard  {Brassica  nigra). 

sepals  and  petals  which  are  borne  on  the  margin  of  a 
well-developed  hypanthium,  formed  by  the  enlargement 
of  the  torus,  at  the  extremity  of  the  peduncle.  The 
numerous  stamens  are  always  inserted  on  the  sepals 
[adnation  of  parts),  and  the  pistils  vary  from  one  to 
many.  In  marked  contrast  to  the  numerous  horticultural 
varieties  of  the  rose,  the  wild  roses  are  single;  that  is,  they 
have  one  circle  of  petals  (usually  five).  The  "doubling" 
of  the  cultivated  varieties  is  caused  by  the  replacement  of 


SEED-BEARING   PLANTS 


465 


stamens  by  petal-like  organs  (Fig.  346).  Not  that 
stamens  are  ''transformed  into  petals,"  as  is  often  stated, 
but  that  petal-like  organs  appear  at  the  points  where 
stamens  normally  occur  in  the  wild  form.  In  other  words, 
the  supernumerary  petals  are  homologous  with  stamens. 


Fig.  348. — Petalody  of  stamens  in  a  cultivated  rose,     a,  indicates  the 
remains  of  anthers  on  petal-like  organs  that  have  replaced  stamens. 


This  homology  is  made  clear  by  transitional  forms,  show- 
ing all  gradations  between  true  stamens  and  normal 
petals  (Figs.  348  and  349). 

By  comparing  the  methods  of  doubling  the  flower  in 
the  buttercup  and  the  rose,  we  see  that  double  flowers  may 
be  produced  by  either  (or  both)  of  two  methods,  often 


\o 


466 


STRUCTURE    AND    LIFE    HISTORIES 


referred  to,  respectively,  as  ''petalody  of  bracts"  and 
''petalody  of  stamens."  The  causes  of  these  variations 
are  not  known. 


Fig.  349. — Wild  rose.  A  "single"  flower  showing  incipient  doubling 
by  the  replacement  of  stamens  by  petals.  Below,  a  series  of  transitional 
forms  from  stamen  to  fully  formed  petal;  an.,  anther,  or  remnant  of  anther. 


Fig.  350.— Perennial  pea  {Lathyrus  latijolius). 

414.  Leguminoseae. — The  legume  family  is  the  largest, 
and  one  of  the  most  widely  distributed  of  all  the  Archi- 
chlamydeae,  and  includes  three  sub-families,  as  follows: 


SEED-BEARING    PLANTS 


467 


I.  FapilionoidecB,    containing    such    commonly    recog- 
nized plants  as  the  peas,  beans,  lentils,  clovers,  peanuts, 


Fig.  351. — Wild  senna  {Cassia  marllandica). 

lupines,     lead-plant     (Amorpha),    locust    or    false-acacia 
(Robinia),  wistaria,   and  others,   all    of  which   have   the 


468 


STRUCTURE    AND    LIFE    HISTORIES 


peculiarly  modified  corolla  called  papilionaceous,  from  its 
fancied  resemblance  to  a  butterfly  (Fig.  350). 

2.  CcBsalpinioidecE,  containing  the  red-bud  {Cercis),  true 
or  honey-locust  (Gleditsia),  wild  senna  {Cassia  marilan- 
dica),  and  others,  whose  flowers  are  only  imperfectly  or 
not  at  all  papilionaceous  (Fig.  351). 

3.  Mimosoidece,  containing  the  acacias,  sensitive  plants 
{Mimosa),  and  others  having  flowers  with  a  regular  corolla. 


Fig.  352. — Legume  of    the  edible  pea   {Pisum  sativum),     a,  anther;  c, 
calyx;  st,  stigma. 


Flowers  that  are  bilaterally  symmetrical,  like  the  papilio- 
naceous flowers,  are  called  zygomorphic.  Such  flowers  as 
the  buttercup,  rose,  and  others  may  be  divided  into  equal 
halves  by  an  infinite  number  of  planes  of  symmetry. 

The  one  feature  that  characterizes  all  three  of  the  sub- 
families^ is  the  simple  pistil,  composed  of  one  carpel,  and 
enlarging  greatly  in  fruit  (as  in  peas  and  beans)  to  form  a 
legume  (Fig.  352);  whence  the  name  of  the  family. 

The  structure  of  the  papilionaceous  corolla  is  illustrated 
in  Fig.  350.     The  upper  and  largest  petal,  stands  erect, 

*  By  some  authors  each  group,  designated  above  as  a  sub-family,  is 
considered  as  a  separate  family. 


SEED-BEARING   PLANTS 


469 


forming  the  standard,  the  two  lateral  petals  are  the 
"wings;"  while  the  two  lower  petals  adhere  along  their 
adjacent  edges  to  form  the  keel.  In  these  flowers  there 
are  usually  ten  stamens  (rarely  live),  which  commonly 
occur  in  two  groups  or  "brotherhoods"  (diadelphous) , 
nine  in  one  group  with  their  filaments  united  into  a  tube, 
cleft  on  the  upper  side,  the  other  standing  alone  above  the 
cleft. 


Fig.  353. — Alsike,  or  Alsatian  clover  {Trijolium  hyhridum).  Inflores- 
cences, showing  carpotropic  movements  of  the  flowers  after  pollination  by 
an  insect.  At  the  extreme  left,  flowers  in  bud,  the  outermost  ones  just 
beginning  to  open;  next  to  the  last,  at  the  right,  only  one  flower  remaining 
erect  on  account  of  not  having  yet  been  pollinated;  at  the  extreme  right, 
every  flower  pollinated,  turned  down,  and  withering. 

Pollination  is  usually  accomplished  in  this  family  by 
means  of  insects  which  visit  the  flowers  for  the  nectar 
secreted  by  glands.  In  the  case  of  white  clover  and 
alsike,  each  flower  of  the  head,  when  pollinated,  turns 
down,  and  the  corolla  becomes  brown  (Fig.  353).  This 
change  has  been  interpreted  by  some  as  a  sign  to  the  insects 
that  the  nectar  has  been  taken,  and  therefore  that  another 


470  STRUCTURE    AND    LIFE    HISTORIES 

visit  would  not  be  profitable.  Other  students  regard  this 
as  questionable. 

Since,  as  shown  by  examining  a  bud,  the  flowers  of  the 
clover  head  mature  from  the  circumference  toward  the 
center  (centripetally),  the  outer  flowers  are  the  first  to  be 
visited,  and  hence  the  first  to  bend  down  (Fig.  353).^ 
Many  of  the  papilionaceous  legumes  are  self-pollinated. 
This  is  true  of  the  ''sweet  pea"  as  a  rule,  but  not  without 
exceptions. 

The  fact  that  the  legumes  furnish  so  many  kinds  of  food 
and  fodder  plants,  and  that  the  organisms  causing  the 
tubercles  on  their  roots  (pp.  317-318)  are  an  important 
source  of  the  nitrogen  necessary  for  successful  agriculture, 
renders  this  family  one  of  the  most  important  of  all  the 
economic  plants,  possibly  exceeded  only  by  the  grass  family. 

415.  Evening-primrose  Family  (Onagraceae). — The 
evening-primroses  have  recently  come  into  very  great 
prominence  on  account  of  the  fact  that  they  have  been 
extensively  used  for  the  experimental  study  of  evo- 
lution. A  knowledge  of  their  structure  has,  therefore  be- 
come increasingly  important.  The  flowers  are  perfect 
and  symmetrical,  with  the  parts  usually  in  fours.  The 
ovary  is  one-  to  six-  (usually  four-)  chambered,  and  the 
calyx  tube  adheres  to  the  walls.  The  stamens  are  in- 
serted commonly  on  the  summit  of  the  calyx  tube.  In 
the  evening-primrose  itself  {(Enothera'^) ,  the  pollen- 
grains  are  held  together  by  delicate  threads  that  resemble 
a  cobweb.  The  seedling  usually  forms  a  rosette  the  first 
year,  and  thus  passes  the  winter  (Fig.  354).     The  follow- 

^  Often  one  will  find  a  solitary,  unpollinated  flower  left  standing,  and  in 
some  localities  these  are  sought  by  children  as  "old-maid  clovers." 
2  Called  also  Onagra. 


SEED-BEARING    PLANTS 


471 


ing  season  an  erect  stem  is  sent  up  which  bears  the  flowers 
and  fruit  (Fig.  400). 

416.  Parsley  Family  (Umbelliferae). — The  most  highly 
organized  family  of  the  Archichlamydeae  is  that  to  which 
the  common  parsley  belongs.     In  this  family  the  calyx- 


HHJI^^^B^  i 

• 

1 

Fig.  354. — Rosette  of  the  evening-primrose  {(Enothera  hwnnis). 

tube  adheres  throughout  its  length  to  the  wall  of  the  ovary, 
and  the  five  petals  and  stamens  are  inserted  on  a  disk 
at  the  base  of  the  two  styles  (Fig.  355).  The  relatively 
small  flowers  are,  with  rare  exceptions,  borne  in  clusters 
known  as   umbels.     The  characteristics  of  an  umbel  are 


472 


STRUCTURE   AND   LIFE   HISTORIES 


that  the  numerous  flower-stalks  are  of  such  relative 
lengths  as  to  bring  all  the  flowers  to  substantially  the  same 
plane  (Fig.  356),  and  that  the  outer  flower  buds  open  first; 
in  other  words  the  anthesis  is  centripetal.     Some  of  the 


Fig.  35S.- 


-Heracleum  lanatum  (Parsley  family), 
the  margin  of  an  umbel. 


Individual  flower  from 


Fig.  356. — A  compound  umbel  of  cow  parsnip  (Heracleum  lanatum). 


genera  of  this  family  {e.g.j  caraway,  parsley,  parsnips, 
carrots)  are  edible,  while,  as  often  happens  in  other 
families  also,  some  of  their  near  relatives  {e.g.,  the  water- 
hemlock,  Cicuta)  are  very  poisonous. 


CHAPTER  XXIX 

SEED-BEARING  PLANTS  (Continued) 

METACHLAMYDEiE  (Sympetalse) 

417.  Coalescence. — We  have  seen  above  that  genera 
normally  having  polypetalous  flowers  frequently  furnish 
examples  of  the  more  or  less  complete  fusion  or  coalescence 
of  the  petals.  In  certain  entire  families  this  fusion  of 
different  members  of  the  same  circle  of  floral  organs  be- 
comes the  rule,  giving  rise  to  an  entire  group,  the  Sym- 
petalcB,  based  upon  this  character.  Only  a  few  of  these 
families  can  be  cited  in  illustration. 

418.  Heath  Family  (Ericaceae). — The  heath  family  in 
North  America  is  composed  chiefly  of  shrubs,  though  a 
few,  as,  for  example,  Indian  pipe  {Monotropa)  are  herbs; ^ 
some  of  the  tropical  genera  are  trees.  The  beautiful 
rhododendrons,  azaleas,  and  laurels,  trailing  arbutus 
(first  harbinger  of  spring  in  the  northern  states) ,  the  aro- 
matic wintergreen,  and  the  well-known  huckleberries 
(Gaylussacia) ,  and  blueberries  {Vaccinium)  belong  here. 

The  structure  of  an  Ericaceous  flower  may  be  illustrated 
by  the  common  mountain  laurel  {Kahnia  latifolia),  of 
the  eastern  states  (Figs.  357  and  358).  The  sepals  are 
united  below,  but  parted  above;  the  sympetalous  corolla 

^  By  some  authors  the  large  heath  family  is  separated  into  a  number  of 
smaller  families,  e.g.,  Monotropaceas,  Ericaceae,  Vacciniaceae.  In  Mono- 
tropaceae,  Clethraceae,  and  Pyrolaceae,  and  a  few  of  the  true  heaths  (Eri- 
caceae), the  corolla  is  polypetalous. 

473 


474 


STRUCTURE    AND    LIFE    HISTORIES 


Fig.  357. — Mountain  laurel  {Kalmia  latifolia).  Note  the  anthers 
resting  in  the  depressions  of  the  sympetalous  corolla.  (Photo  by  Elsie 
M.  Kittredge.) 


Fig.  358. — Mountain  laurel  {Kalmia  latifolia).  a,  stamens  in  their 
original  position,  with  the  anthers  in  the  pouches;  b,  stamens  inflexed 
(detail  at  e);  c,  side  view;  d,  essential  organs;/  and  g,  stamens. 


SEED-BEARING    PLANTS 


475 


is  five-lobed  having  the  shape  of  a  broad,  shallow  bell.  Its 
coloration^  as  viewed  en  masse  on  the  bush,  gave  rise  to 
the  common  name  "calico  bush,"  popular  in  certain  locali- 


Fig.  359. — Common  milkweed,  Asclepias  syriaca. 

Kittredge.) 


(Photo  by  Elsie  M. 


ties.  The  corolla  has  ten  pouches,  each  of  which  con- 
tains one  anther  borne  on  a  recurved  filament.  When 
an  insect  alights  on  the  flower  the  anthers  become  loosened 

^  Coloration  =  color  pattern. 


476 


STRUCTURE    AND    LIFE    HISTORIES 


from  the  pouches,  and  snap  over  in,  toward  the  center, 
thus  dusting  the  insect  with  pollen,  which  is  then  trans- 
ferred to  the  stigma  of  the  next  flower  visited  by  the 
insect. 

The  corpse-plant,  or  Indian-pipe  {Monotropa  uniflora),  is 
of  interest  because,  although  belonging  in  a  sympetalous 


Fig.  360. — Milkweed  {Asdepias  sp.).  a,  flower-bud;  b,  flower;  c, 
very  young  pod;  d,  older  pod  in  section,  showing  seeds;  e,  section  of 
flower;  /,  top  view  of  flower,  showing  the  5  Jioods  of  the  crown,  each  with 
a  horn  incurving  to  the  stigma;  between  the  horns  are  the  cleft  glands 
(shown  enlarged  at  g),  to  which  the  pollinia  are  attached. 

family,  it  is  polypetalous,  and  further  because,  Hving 
entirely  as  a  saprophyte  (or,  possibly,  as  a  root-parasite),  it 
has  entirely  lost  the  power  to  make  chlorophyll,  and  hence 
the  power  of  photosynthesis  (Fig.  230,  p.  323). 

419.  Milkweed  Family  (Asclepiadaceae). — The  milk- 
weeds^ present  a  most  curious  and  interesting  modifica- 

^  So  called  because  they  contain  a  milky  juice  or  latex. 


SEED-BEARING    PLANTS  477 

tion  of  flower-structure  (Fig.  359).  The  deeply  five-parted 
and  reflexed  corolla  bears  a  crown  of  five  "hooded"  bodies, 
in  each  of  which  there  arises  a  pointed,  incurved  ''horn" 
(Fig.  360).  The  anthers  are  more  or  less  united  around  the 
stigma,  and  each  cell  contains  a  waxy,  pear-shaped  pollen- 
mass  (pollinium).  The  pollinia  of  adjacent  anthers  adhere 
in  pairs  to  cleft  glands  that  grow  one  on  each  of  the  five 
angles  of  the  stigma.  As  bees  climb  over  the  flowers  in 
search  of  nectar  in  the  bottom  of  the  hoods,  their  legs 
are  drawn  through  tiny  slits,  and  catch  the  cleft  gland 


Fig.  361. — Milkweed     (Asclepias).     Pollen-mass     (pollinuim),    showing 

germination. 


when  pulled  out.  Often  the  gland  cannot  be  loosened, 
and  the  legs  of  the  insects  are  pulled  ofl  and  left  attached 
to  the  flower.  When  the  insect  visits  another  flower  the 
pollen-masses  (which  by  this  time  have  twisted  and 
folded  together)  become  inserted  into  the  stigmatic 
chamber  of  the  second  flower,  where  they  germinate, 
sending  out  numerous  pollen- tubes  (Fig.  361).  On  ac- 
count of  the  complicated  nature  of  this  process,  pollina- 
tion often  fails,  so  that  only  a  small  percentage  of  the  very 
numerous  flowers  produces  seed.  After  fertilization,  the 
carpels  increase  enormously  in  size,  and  ripen  into  a  pod, 
filled  with  a  large  quantity  of  flat,  thin  seeds,  each  of 


478  STRUCTURE    AND    LIFE    HISTORIES 

which  bears  a  tuft  of  long  silky  hairs,  whence  one  of  the 
common  names,  ''silk  weed."  These  hairs  facilitate  the 
distribution  of  the  seeds  by  the  wind  (Fig.  362). 


Fig.  362. — A  milkweed   {Asclepias  syriaca),  with  tufted  seeds  scattering 
from  the  dehiscing  pods.     (Photo  by  Elsie  M.  Kittredge.) 

420.  Convolvulus  Family  (Convolvulaceae). — The  fea- 
tures of  this  family  are  well  illustrated  by  the  genus 
from  which  it  gets  its  name,  Convolvulus,  or  bind-weed. 
The  five-lobed  corolla  is  bell-shaped,  all  the  parts  of  the 
flower  are  in  fives,  and  the  pistil  two-celled.  Most  genera 
of  the  family  are  trailing  or  (as  the  name  indicates) 
twining  vines  (Fig.  363). 


SEED-BEARING    PLANTS 


479 


421.  Mint  Family  (Labiatae).— The  Mint  family  is 
characterized  by  a  square  stem,  opposite  leaves,  a  tubular 
calyx,  caused  by  the  coalescence  of  the  five  sepals,  a  highly 
modified  corolla  having  two  lips  or  labia  (singular  labium, 
whence  the  family  name),  and   leaves  containing  many 


Fig.  363. — Bindweed  {Convolvulus  arvensis). 


small  glands  that  secrete  a  volatile  oil,  which  gives  the  char- 
acteristic odor  and  taste  to  all  the  plants  of  the  family. 
The  upper  lip  results  from  the  fusion  of  two  petals,  the 
lower  lip  by  the  fusion  of  three.  Everyone  is  familiar 
with  one  or  more  of  these  features,  as  embodied  in  the 
various  mints,  pennyroyal,  horehound,  catnip,  sage, 
savory,  thyme,  hyssop,  wild  marjoram,  and  other  condi- 


48o 


STRUCTURE    AND    LIFE    HISTORIES 


ments  and  drugs.     The  features  of  the   flower  are  illus- 
trated in  Fig.  364. 

422.  Nightshade    Family    (Solanaceae).— The    Night- 
shade   family  is  of   interest    chiefly   because  it  contains 


Fig.  364. — Salvia  sp.  (One  of  the  Labiatae).  a,  flower  bud;  b-f, 
various  views  of  the  open  flower;  an.,  anther;  st.,  stigma;  x,  projections 
near  the  base  of  the  filaments.  The  lead  pencil  is  made  to  imitate  an 
insect  visiting  the  flower  for  pollen.  By  pressure  at  the  base  of  the  fila- 
ments, the  anthers  are  brought  into  contact  with  the  surface  of  the  pencil, 
which  thus  becomes  covered  with  pollen.  When  the  next  flower  is  visited 
the  stigma,  having  then  bent  down  and  spread  apart,  receives  the  pollen 
from  the  other  flower.  Thus  is  accomplished  cross-pollination.  In  h, 
before  the  visit  of  the  insect,  the  stigmatic  surfaces  are  still  in  contact,  so 
that  pollination  is  not  possible. 

several  genera  of  very  great  economic  importance,  viz.: 
potato  {Solamim  tuberosum)  jtohsicco  {Nicotiana  Tahacum), 
tomato    (Lycopersicum    esculentum)^    and    several   medi- 


SEED-BEARING   PLANTS 


481 


cinal  herbs,  such  as  belladonna,  henbane,  capsicum,  and 
others.  Most  of  the  genera  are  tropical.  The  bitter- 
sweet {Solanum  Dulcamara^)  may  serve  to  illustrate  the 
family  (Fig.  365).  This  is  a  perennial,  climbing  vine,  hav- 
ing the  Darts  of  the  flower  (including  the  stamens)  in  fives 


Fig.  365. — Nightshade,  or  bittersweet  {Solanum  Dulcamara). 

and  the  fruit  a  two-celled  herry  with  many  seeds  {e.g.,  the 
tomato.  Fig.  334).  The  corolla  is  wheel-shaped  and  five- 
parted,  and  the  anthers  converge  to  form  a  tube  around  the 
single  style.  The  anthers  discharge  the  pollen  through  a 
pore  or  ''chink"  at  the  tip. 

423.  Figwort  Family  (Scrophulariaceae). — The  ''butter- 
and-eggs, "  or  "toadflax"  {Linaria  vulgaris),  will  serve  to 
illustrate  the  figworts  (Fig.  366).  The  stamens  are  inserted 

^  The   staff- tree   or   waxwork    {Celastrus  scandens),   of   the   Staff- tree 
family,  Celastraceae,  is  also  called  "bitter  sweet"  in  certain  localities. 
31 


482  STRUCTURE    AND    LIFE    HISTORIES 

on  the  tube  of  the  irregular,  two-lipped  corolla,  which 
bears  a  well-developed  spur  at  the  base  (Fig.  367).  Not 
infrequently  abnormal  flowers  are  found  with  five  spurs, 


F'iG.  366. — The  toad-flax,  or  butter-and-eggs  {Lhiaria  vulgaris). 

or  with  none,  and  other  attendant  modifications  of  the 
corolla  (Figs.  368  and  369).  Such  flowers  are  called 
pelories,  since  they  are  thought  to  be  variations  indicating 
the  character  of  the  ancestral  form  from  which  they  are 


SEED-BEARING   PLANTS 


483 


believed  to  be  derived.     The  method   of  pollination  in 
normal  flowers  is  illustrated  in  Fig.  370. 

424.  Composite  Family  (Compositae). — The  composites 
represent  the  highest  development  of  dicotyledons.  The 
flowers  are  borne  on  a  common  receptacle  in  a  compact 


Fig.  367. — Sections  of  flowers  of  the  toad-flax  (Linaria  vulgaris). 
A,  front  view;  a,  anthers;  s,  stigma;  n,  nectar-gland.  B,  side  view;  o, 
ovary. 


head,"  giving  the  appearance,  not  so  much  of  an  in- 
florescence as  of  a  compound  flower,  whence  the  family 
name,  assigned  by  early  botanists,  who  did  not  understand 
the  morphology  of  the  head. 

The  heads  are  surrounded  by  a  circle  of  bracts,  called 
an  involucre.     The  bracts  which  often  occur  on  the  re- 


484 


STRUCTURE   AND    LIFE    HISTORIES 


ceptacle  among  the  flowers  are  called  chaf.     The  family  is 
composed  of  two  series:  I.  Tubuliflorae,  with  all  the  perfect 


Fig.  368. — Toad- flax  {Linaria  vulgaris).     Inflorescence,  showing  normal 
(spurred)  flowers,  and  several  abnormal  flowers  without  spurs. 

flowers  tubular,  as  in  the  daisy;  II.  Liguliflorae,  with  no 
tubular  flowers,  but  all  the  flowers  having  a  strap-shaped 


SEED-BEARING   PLANTS 


48s 


Fig.  369. — Toad-flax   (Linaria  vulgaris).     Abnormal   flowers    (pelories), 
having  five  or  more  spurs.     Normal,  one-spurred  buds  are  shown  in  A . 


Fig.  370. — Toad-flax  {Linaria  vulgaris).  Flowers  being  visited  by  an 
insect  for  nectar.  B,  longitudinal  section,  showing  the  insect's  proboscis 
extended  down  the  spur  toward  the  nectar-gland;  C,  insect  with  a  mass  of 
pollen  ip),  rubbed  off  from  anthers  onto  the  dorsal  hairs  of  the  thorax, 
during  successive  visits. 


486 


STRUCTURE    AND    LIFE    HISTORIES 


{ligulate)  corolla,  formed  by  the  fusion  of  the  five  petals, 
as  indicated  by  the  five  notches  at  the  end  (Fig.  372). 
The  tubuliflorae  may  have  both  tubular  and  ligulate 
flowers,  as  in  boneset  (Eupatorium  perjoliatum)  or  in  white 
daisy  {Chrysanthemum  leucanthemum)  ^  or  only  tubular,  as 
in  the  burdock  {Arctium,  Fig.  371),  or  in  the  Canada  thistle 


Fig.  371. — Inflorescence  and  flowers  of  the  burdock  {Arctium  minus), 
a,  Inflorescences;  h,  longitudinal  section  of  the  same;  c,  bud  of  individual 
flower;  d,  mature  flower;  sty,  stigma;  stig,  style;  a,  ring  of  syngenesius 
anthers;  c,  corolla;  p,  pappus  (calyx);  ov,  ovary;  e,  mature  seed. 

{Cirsium  arvense).  Among  the  liguliflorag  may  be  men- 
tioned chicory  {Cichorium  Intyhus,  Fig.  372),  dandelion 
{Taraxacum),  garden  lettuce  {Lactuca  sativa),  and  others, 
all  of  which  possess  a  milky  juice,  or  latex.  The  five  stamens 
(rarely  four)  are  inserted  on  the  corolla,  and  have  their 
anthers  united  in  a  tube  {syngenesious)  around  the  style. 


SEED-BEARING   PLANTS 


487 


The  calyx  tube  is  united  to  the  one-celled  ovary,  and 
its  upper  free  part,  or  limb,  is  dififerentiated  into  hairs 
{pappus),  scales,  teeth,  or  is  merely  cup-shaped,  or  in  some 


Fig.  372. — Chicory  {Cichoriiim  Intyhus).  A,  portion  of  flowering 
branch;  B,  basal  leaf  (runcinate-pinnatifid) ;  C,  median  longitudinal  section 
through  a  head,  showing  the  insertion  of  the  flowers;  D,  individual  flower; 
E,  fruit  (ripened  ovary),  showing  the  persistent  pappus  (calyx)  of  short 
scales. 


species   entirely  wanting.     When   the   fruit   is   ripe,  the 
pappus  aids  in  its  dissemination  by  the  wind. 

While  the  style  is  growing  up  through  the  tube  of  anthers 
the  stigmatic  surfaces  are  in  contact  with  each  other,  and 


488 


STRUCTURE    AND    LIFE   HISTORIES 


thus  protected  from  becoming  pollinated  with  the  flower's 
own  pollen.  As  the  styles  emerge  above  the  anthers  their 
two  tips  spread  apart  and  roll  back,  exposing  the  infacing 


Fig.  373. — A  composite  {Coreopsis  sp.).  A,  B,  E,  views  of  the  inflor- 
escence or  head;  C,  a  ray-flower;  D,  section  through  the  head;  F,  a  disc- 
flower in  bud;  G,  disc-flower  just  opened;  H,  older  disc-flower,  the  stigmas 
reflexed;  /,  disc-flower  with  corolla  removed. 

stigmatic  surfaces  so  that  they  may  receive  pollen  brought 
by  insects  from  other  flowers  (Fig.  373).^ 

^  The  family  Compositae,  as  recognized  above,  including  two  series, 
Tubuliflorae  and  Liguliflorae,  is  restricted  by  some  authors  so  as  to  include 
only  plants  having  florets  with  tubular  or  both  tubular  and  ligulate  cor- 
ollas in  the  head.  Such  plants  as  the  chicory,  dandelion,  and  lettuce, 
having  only  ligulate  corollas,  comprise  the  family  Cichoriaceae.  Cichor- 
iaceae  and  Compositae  (in  this  restricted  sense)  have  syngenesious  anthers. 
Plants  whose  florets  have  tubular  corollas  only  (sometimes  none),  but 
anthers  not  truly  syngenesious,  comprise  the  Ambrosiaseae,  including  the 
rag- weeds,  cockle-bur,  marsh-elder  (Iva),  and  GcBrtneria. 


CHAPTER  XXX 

SEED-BEARING  PLANTS  (Concluded) 

MONOCOTYLEDONS 

425.  General  Characters. — The  monocotyledons  are, 
in  almost  every  respect,  of  simpler  structure  than  the 
dicotyledons.  As  the  name  indicates,  the  embryo  has 
only  one  cotyledon;  the  parts  of  the  flower  are  usually 
in  threes  or  sixes,  but  never  in  fives,  as  in  dicotyledons; 
the  leaves  are,  with  rare  exceptions,  parallel-veined, 
and  the  early  ones  are  always  alternate  on  the  stem.  A 
cross-section  of  the  stem  shows  that  the  fibro-vascular 
bundles  are  not  arranged  in  a  circle  about  a  central 
pith  (exogenous  type),  but  are  distributed  irregularly 
throughout  the  parenchyma  (endogenous).  There  is  no 
layer  of  perennial  cambium,  and  consequently  no  cylinders 
of  wood  and  bark  are  formed  each  growing  season,  as  in 
the  dicotyledons.  The  general  characters  of  the  group 
are  illustrated  diagrammatically  in  Fig.  ^;^S. 

426.  Relation  to  Dicotyledons. — A  comparison  of  the 
monocotyledons  with  the  more  highly  developed  dicoty- 
ledons raises  at  once  the  question  as  to  whether  the  former 
are  the  more  ancient  forms  from  'which  the  dicotyledons 
have  been  evolved,  or  whether  dicotyledons  are  the  more 
primitive,  in  order  of  development,  and  the  monocotyle- 
dons derived  from  them  by  reduction  and  simplification. 
There  is  evidence  on  both  sides  of  this  question,  which  will 

489 


490 


STRUCTURE    AND    LIFE    HISTORIES 


be  considered  again  later  on  (page  608).     In  treating  of 
the  dicotyledons  first,  however,  we  have  tacitly  assumed 


YiG^  374.— Cat-tails.     Typha  angustijolia  at  left;  Typha  latifolia  at  right. 
Staminate  flowers  above;  pistillate  flowers  below. 

that  they  are  the  older    forms,   from  which  the  mono- 
cotyledons have  been  derived. 


SEED-BEAEING    PLANTS  49I 

TYPES  OF  MONOCOTYLEDONS 

427.  Cat-tail  Family  (Typhaceae).— The  cat- tails  are 
one  of  the  most  primitively  organized  family  of  Angio- 
sperms.  They  grow  in  groups  or  associations  in  swamps 
and  wet  places,  and  are  conspicuous  by  their  tall,  upright 
and  grass-like  leaves,  and  by  the  inflorescences  or  *' cat- 
tails"  in  late  summer  and  fall  (Fig.  374).     The  monoecious 


Fig.  375. — Staminate  flower  of  the  broad-leaved  cat-tail   {Typha  lati- 
folia).     (Cf.  Fig.  376.) 


flowers  are  borne  on  a  fleshy  axis  or  spadix,  the  pistillate 
below,  and  the  staminate  above  near  the  tip.  The  flowers 
have  neither  calyx  nor  corolla  (Figs.  375  and  376),  and  the 
one-celled  ovary  contains  but  one  ovule.  The  embryo  is 
surrounded  by  abundant  endosperm,  often  called  albu- 
men. The  copious  brown  down  of  the  fruit  is  composed 
of  club-shaped  hairs  on  the  stipe,  or  stalk-like  support  of 
the  pistil. 

428.  Water-plantain  Family  (Alismaceaej. — The  type 
genus^  of  the  water-plantain  family  is  the  water-plantain 

^  The  type  genus  of  a  family  is  the  genus  from  which  the  family  name 
.-,ften  is  derived,  and  usually  (though  not  always)  embodies  in  a  striking 
way  the  characters  which  distinguish  the  family. 


492 


STRUCTURE    AND    LIFE   HISTORIES 


(Alisma  Plantago-aquatica) .  Like  the  cat-tails,  the  water- 
plantains  are  marsh  herbs  (Fig.  377),  with  flowers  either 
perfect,  monoecious  or  dioecious;  in  Alisma  they  are  per- 
fect, with  usually  six  stamens.  The  three  sepals  are  per- 
sistent^ but  the  three  white  petals  are  deciduous  (i.e.,  falling 


Fig.  376. — Cat-tail  {Typha  latifolia).  A,  longitudinal  section  of 
portion  of  inflorescence;  Sp,  spadix;  p.f.,  pistillate  flowers;  s.f.,  staminate 
flowers.  B,  pistillate  flower,  greatly  magnified;  ov,  ovary;  sty,  style;  stig., 
stigma;  sc,  sterile  hair;  P,  pollen  grains,  in  characteristic  groups  of  four 
each.     (Cf.  Fig.  375.) 

away  early).  The  numerous  ovaries  are  borne  in  a  circle 
on  a  flattened  receptacle.  The  possession  of  calyx  and 
corolla,  together  with  other  features,  mark  the  family  as 
more  highly  organized  than  the  cat-tails. 

429.  Grass  Family  (Gramineae).— The  grasses  consti- 
tute one  of  the  largest,  one  of  the  most  important  econom- 
ically, and  one  of  the  most  difiicult  taxonomically,  of  all 


SEED-BEARING   PLANTS 


493 


the  plant  families.  Here  belong  the  *^  grains ''  (wheat,  rice, 
oats,  barley,  rye,  corn,  and  others)  which  furnish  the  most 
important  vegetable  foods  of  the  entire  human  race.  The 
commercial  grains  have  been  cultivated  so  long  by  man 


Fig.  2>11' — Water  plantain  {Alisma  Plantago-aqiiatica) . 

that  their  origin  is  shrouded  in  mystery,  antedating,  as  it 
does,  the  dawn  of  written  history.  Except  the  bamboos, 
which  are  shrubs  or  trees,  the  grasses  are  all  herbaceous. 

The  chief  characteristics  of  the  family  are  that  the  fruit 
is  always  a  grain  or  caryopsis}     The  seed  invariably  con- 

^  A  one-seeded,  seed-like  fruit,  with  the  wall  of  the  ovary  {pericarp) 
united  closely  (adnate)  to  the  seed  within. 


494 


STRUCTURE    AND    LIFE    HISTORIES 


tains  endosperm,  with  the  embryo  located  at  one  side 
(Figs.  378  and  83).  The  cotyledon  {sciitellum)  serves  to 
secrete   enzymes   that   digest   the   endosperm,   and   then 


Fig.  378.-Longitudinal  section  of  a  grain  of  wheat  {Triticum  vulgare), 
p.s.c,  pericarp  and  seed-coats  (united);  en,  endosperm;  al,  aleurone  layer; 
gl,  glandular  layer  of  the  scutellum;  sc,  scutellum;  sp,  sheath  of  plumule 
{coleoptile);  pi,  plumule;  r.cot.,  rudimentary  cotyledon;  ra,  radicle;  re, 
root-cap;  r.s.,  root-sheath  (coleorhiza).  (From  microscopical  preparation 
of  E.  W.  Olive.) 


absorb  the  digested  food  by  osmosis  (Fig.  62).  The  cylin- 
drical stems  are  jointed  and  usually  hollow  (except  at  the 
joints) .     The  leaves  are  usually  long,  slender,  and  pp.rallel- 


SEED-BEARING    PLANTS 


495 


veined.  The  flowers  are  arranged  in  s pikelets,  and  the 
spikelets  in  spikes  (Fig.  379),  racemes,  or  panicles^  (Fig. 
380).  The  leafy  parts  of  the  flower-clusters 
are  modified  as  dry  scales  called  glumes. 
When  grain  is  thrashed  the  glumes  consti- 
tute the  so-called  ''chaff."  There  is  no 
perianth. 

430.  Palm  Family  (Palmaceae).  — The 
palms  are  mostly  tropical  and  subtropical. 
With  certain  exceptions  {e.g.,  Calamus — rat- 
tan), the  stem  or  caudex  is  normally  un- 
branched,  varies  in  height  with  the  species,  and 
in  most  species  bears  all  the  foliage  near  the 
tip  (Figs.  381  and  382).  Frequently  the  old 
leaves,  or  their  petioles  or  bases  only,  re- 
main attached  to  the  trunk.  The  leaves, 
though  not  morphologically  compound,  usu- 
ally have  their  blades  cleft  or  divided  as  they 
mature.  The  flowers  are  complete,  with  three 
sepals  and  petals,  three  to  six  stamens,  and 
pistil  of  three  carpels.  They  are  monoecious, 
dioecious,  or  perfect,  according  to  the  species. 

The  flowers,  intermingled  with  bracts,  oc-  yig.  379.— 
cur  on  a  more  or  less  fleshy  spadix,  often  Naziaracemosa 

111.;  ,         (L.)       Kuntze. 

much  branched,  and  enclosed  m  a  large  spathe.  Terminal  spike 

The  inflorescence  may  be  axial  (lateral),  or  'l^^^  l^i^^x^;, 

terminal.     When  terminal  the  plant  usually  (After  Britton 
dies  after  the  fruit  has  matured. 

^  A  spike  is  "a  form  of  simple  inflorescence  with  the  flowers  sessile  or 
nearly  so  upon  a  more  or  less  elongated  common  axis."  A  panicle  is  "a 
loose,  irregularly  compound  inflorescence  with  pedicellate  flowers."  A 
raceme  is  "a  simple  inflorescence  of  pediceled  flowers  upon  a  common,  more 
>r  less  elongated  axis." 


496 


STRUCTURE    AND    LIFE    HISTORIES 


No  species  of  palm  has  ever  been  found  in  both  the 
eastern  and  western  hemisphere,  except  when  introduced 
artificially.  Many  of  the  palms  have  great  commercial 
value,  such  as  the  date  palm,  cocoanut  palm  (Fig.  383), 
fan  palm,  vegetable-ivory  palm  (whose  endosperm  is 
hard  and  white  like  ivory),  and  the  oil  palm.  In  the  trop- 
ics the  leaves  of  various  species  are  used  to  make  thatched 


Fig.  380. — Johnson-grass   {Sorghum   halpense).     Spikelets  in   a  panicle. 
(After  Britton  and  Brown.) 


roofs,  and  the  trunks  are  often  used  for  fence  posts  and 
porch  pillars. 

431.  Arum  Family  (Araceae). — The  arum  family  is 
well  illustrated  by  the  "skunk  cabbage"  {Symplocarpiis 
fcetidiis)  (Figs.  384  and  385).^  The  flower  has  no  petals, 
but  four  sepals,  and  four  stamens — one  opposite  each 
sepal.  The  ovary  contains  only  one  suspended  ovule. 
The  compound  globular  fruit  is  composed  of  the  spongy 
spadix,  greatly  enlarged,  bearing  the  coalesced  ovaries, 
with   the   spherical   seeds   just  underneath   the   surface. 

^  Called  by  some  authors,  Spathyema  fcetida. 


SEED-BEARING   PLANTS 


497 


The  roughness  of  the  surface  is  caused  by  the  persistence 
of  the  styles  and  the  fleshy  sepals. 

The  family  is  divided  into  several  sub-families  on  the 
basis  of  various  structural  differences,  such  as  phyllotaxy, 
venation  of  leaves,  presence  or  absence  of  milky  juice, 
presence  or  absence  of  perianth,  and  others. 


"^ 

^ 

P 

^ 

\      4^ 

m,.^ 

, ,     ^ 

m 

''%m\ 

H  ;  S 

m 

:=;is^^  ■• 

m 

. "..:  ^ -^:^::3-^-1m 

4 

W 

'•'""" 

Fig.  381. — Cocoanut  palms  along  the  beach.     Philippine  Islands. 
(Photo  from  Bureau  of  Science,  Manila.) 

432.  Orchid  Family  (Orchidaceae). — The  Orchidaceae 
are  the  most  highly  developed  of  all  monocotyledons. 
No  flower  surpasses  the  orchids  in  beauty  of  color- 
pattern,  endless  diversity  of  unusual  forms,  and  wonder- 
ful mechanisms  that  secure  cross-pollination  by  insects. 
As  in  the  papilionaceous  flower,  the  flower  of  orchids  is 
bilaterally  symmetrical  (zygomorphic),  with  (usually) 
three  sepals  resembling  in  texture  the  three  petals  (Fig. 

J2 


498 


STRUCTURE    AND    LIFE    HISTORIES 


386).  One  of  the  petals,  the  Up,  presents  a  greater  variety 
of  form  in  the  various  species  than  do  the  other  petals.  At 
its  base  is  the  column,  composed  of  the  style,  with  which 
are  fused  the  one  (or  sometimes  only  two)  stamens  (Fig. 
387).     Except  in  the  lady's  slipper  {Cypripedium),  and  its 


Fig.  382. 


-Sabal   palmetto.   (In  the  right  distance  a  barragona  palm). 
Cuba. 


nearest  relatives,  the  pollen  adheres  in  masses  or  pollinia, 
as  in  the  milk-weed.  The  stalked  pollinia  adhere  to  visit- 
ing insects,  sometimes  to  their  eyes,  and  are  thus  trans- 
ferred from  one  flower  to  another.^ 


^  For  details  of  the  wonderful  contrivances  for  cross-pollination,  the 
student  should  consult  some  larger  treatise,  such  as  Darwin's  "Cross' 
fertilization  of  Orchids." 


SEED-BEARING    PLANTS 


499 


Fig.  383. — Germinating  cocoanuts;  the  one  seeded  fruit  of  the  cocoanut 
palm.     (From  Bull.  Agricole  du  Congo  Beige,  1910.) 


Fig.  384. — Skunk  cabbage  {Symplocarpiis  fcctidus).     Early  spring  growtk 
and  flower  buds.     (Photo  by  Elsie  M.  Kittredge.) 


500  STRUCTURE   AND    LIFE   HISTORIES 


Fig.  385.^ — Skunk  cabbage  {Symplocarpus  Joetidus).  Inflorescence, 
with  portion  of  the  fleshy  spathe  removed,  showing  the  perfect  flowers 
covering  the  globose  spadix.     (Photo  by  Elsie  M.  Kittredge.) 


Fig.  386.— Flower  of  an  orchid  {Cattleya  sp.).     (Cf.  Fig.  387.) 


SEED-BEARING    PLANTS 


501 


Of  about  24,000  known  species  of  monocotyledons, 
over  one-fourth  are  orchids.  The  number  of  individuals, 
however,  of  any  given  species  is  small,  as  compared,  for 


Fig.  387. — Floral  organs  of  an  orchid  {Catlleya  sp.).  A,  the  entire 
flower;  sep,  sepal;  pet,  petal;  B,  column,  showing  5,  stigma  and  r,  the  ros- 
tellum  (beak),  with  the  small  glands  at  the  tip;  to  the  glands  are  attached 
the  four  strap-shaped  caudicles  of  the  poUinia;  C,  pollinia,  with  the  four 
caudicles;  below,  the  gland;  D,  longitudinal  section  of  the  column;  p, 
poUinium;  E,  the  same,  enlarged.     (Cf.  Fig.  386,) 

example,  with  the  grasses.  The  most  highly  modified 
forms  are  tropical,  and  are  seen  in  temperate  regions 
only  in  plant-houses  ~ 


CHAPTER  XXXI 
EVOLUTION 

433.  Doctrine  of  Special  Creation. — In  the  time  of 
Linnaeus,  the  ''father  of  botany/'  men  believed  that  the 
seven  *'days"  of  creation  left  the  world  substantially  as 
we  now  find  it.  The  stars  and  planets,  mountains  and 
oceans,  plants  and  animals  were  created  once  and  for  all, 
and  continued  without  important  change  until  the  present. 
In  the  beginning,  as  now,  there  were  the  same  oceans  and 
hills,  the  same  kinds  of  plants,  and  the  same  kinds  of 
animals.  Nor,  it  was  believed,  are  any  fundamental 
changes  now  in  progress.  Creation  was  not  continuous; 
it  took  place  within  a  brief  period  (seven  ''days"),  and 
then  ceased;  after  that  the  Creator  merely  watched  over 
the  objects  of  his  handiwork. 

434.  Meaning  of  Evolution. — Evolution  means  gradual 
change.  Applied  to  the  natural  world  the  theory  of 
evolution  is  the  direct  opposite  of  the  doctrine  of  special 
creation.  It  teaches  that  things  were  not  in  the  beginning 
as  we  now  find  them,  but  that  there  has  been  constant 
though  gradual  change.  Creation  is  regarded,  not  as 
having  taken  place  once  and  for  all,  but  as  being  a  con- 
tinuous process  operating  from  the  beginning  without 
ceasing — and  still  in  progress. 

435.  The  Course  of  Evolution. — The  theory  teaches 
that  the  gradual  changes  have  been  from  relatively 
simple   conditions   to   those   more   complex.     The   com- 

502 


EVOLUTION  503 

plication  has  been  two-fold:  (i)  simple  individuals, 
whether  mountains,  rivers,  planets,  animals,  or  plants,  have 
become  more  complex  {e.g.,  compare  the  structure  of 
Pleurococcus,  a  simple  spherical  cell,  with  that  of  the  fern) ; 
(2)  the  relation  between  living  things,  and  between  them 
and  their  surroundings  has  become  more  complex  {e.g., 
compare  a  unicellular  bacterium,  with  its  relatively  simple 
life  relations,  with  the  clover  plant,  highly  organized, 
and  related  to  water,  air,  soil,  light,  temperature,  gravity, 
bacteria  (in  its  roots),  and  insects  (for  cross-pollination). 

Most  of  the  steps  of  evolution  have  been  progressive, 
toward  higher  organization,  greater  perfection  of  parts, 
increased  efficiency  of  function,  as,  for  example,  from  algae 
to  angiosperms;  but  not  all  the  steps  have  been  in 
this  direction.  Some  of  the  steps  have  been  regressive, 
toward  simpler  organization,  less  perfection  of  parts, 
decreased  efficiency  of  function,  as,  for  example,  from 
green  algae  to  the  alga-like  fungi  (Phy comycetes) ,  from 
independence  to  parasitism  (dodder),  or  to  saprophytism 
(Indian  pipe  and  bread-mold). 

436.  Inorganic  Evolution. — The  process  of  evolution  is 
not  confined  to  living  things,  but,  as  indicated  above, 
applies  to  all  nature.  Even  the  chemical  elements  are 
now  believed  to  have  been  produced  by  evolutionary 
changes,  and  to  be  even  now  in  process  of  evolution.  This 
is  one  of  the  results  of  the  recently  discovered  phenomenon 
of  radioactivity,  which  is  essentially  the  transformation 
of  the  atoms  of  one  chemical  element  into  those  of  another. 
Fossil  remains  of  marine  animals  and  plants,  found  im- 
bedded in  the  rocks  on  mountain  summits,  indicate, 
without  possibility  of  reasonable  doubt,  that  what  is  now 
mountain  top  was  formerly  ocean  bottom.     The  mountain 


504  STRUCTURE    AND    LIFE   mSTORIES 

has  come  to  be,  by  a  series  of  gradual  changes.  Rivers 
and  valleys  are  constantly  changing  so  that  the  present 
landscape  is  the  result  of  evolutionary  processes;  climates 
have  changed,  as  we  know  from  the  fact  that  fossil  re- 
mains of  tropical  plants  are  now  found  in  the  rocks  in 
arctic  regions;  even  the  stars  and  planets,  like  our  own 
earth,  are  coming  gradually  into  being,  undergoing  changes 
of  surface  and  interior  condition,  and  ceasing  to  exist. 
Nothing  is  constant  except  constant  change.  The  main 
problem  of  astronomy  is  to  ascertain  and  record,  in  order, 
the  evolutionary  changes  that  have  resulted  in  the  present 
system  of  suns  and  planets.  The  main  problem  of 
geology  is  to  ascertain  and  record,  in  order,  the  evo- 
lutionary steps  that  have  resulted  in  the  present  condition 
of  the  earth. 

437.  Organic  Evolution. — Developmental  changes  in 
living  things  constitute  organic  evolution.  Such  changes 
are  manifested  in  the  development  of  an  individual  from 
a  spore  or  an  Qgg.  The  development  of  a  mature  in- 
dividual is  ontogeny.  The  development  of  a  group  of 
related  forms  (genera,  families,  orders,  etc.)  is  phytogeny. 
The  chief  problem  of  biology  is  to  ascertain  and  record, 
in  order,  the  evolutionary  changes  that  have  resulted  in 
the  appearance  of  life  and  the  present  condition  of  living 
things. 

The  major  problem  of  botany  is  to  record,  in  order,  the 
evolutionary  steps  that  have  culminated  in  the  present  con- 
dition of  the  plant  world. 

Organic  evolution  means  that,  after  the  first  appearance 
of  life,  all  living  things,  plant  or  animal,  have  been 
derived  from  preexisting  living  things,  in  other  words,  that 
the  present  method  of  formation  of  living  things,  by  the 


EVOLUTION 


505 


reproduction  of  organisms  already  existing,  has  always 
been  the  method — ^'Omne  vivum  ex  ovo^^  (all  life  from  an 
egg))  ^^omne  vivum  e  vivo''  (all  life  from  preexisting  life). 
438.  Method  of  Evolution. — To  recognize  that  evolution 
is  the  method  of  creation  still  leaves  unanswered  the  im- 
portant question  as  to  the  method  of  evolution.  By  what 
process  was  the  gradual  development  of  the  living  world 
accomplished?  Various  hypotheses  have  been  elaborated 
in  answer  to  this  question.  We  can  here  only  briefly 
outline  three  of  the  most  important  ones. 


Fig.  388.— Louis  Agassiz.     (From  Ballard's  "Three  Kingdoms.") 


I.  Agassiz' s  Hypothesis. — The  great  teacher  and  student 
of  nature,  Louis  Agassiz,  beheved  that  the  vast  array  of 
plant  and  animal  species,  past  and  present,  had  no  material 


5o6  STRUCTURE    AND    LIFE    HISTORIES 

connection,  but  only  a  mental  one;  that  is,  they  merely  re- 
flected the  succession  of  ideas  as  they  developed  in  the 
mind  of  the  Creator,  but  were  not  genetically  related  to 
each  other.  ''We  must  .  .  .  look  to  some  cause  outside 
of  Nature,  corresponding  in  kind  to  the  intelligence  of 
man,  though  so  different  in  degree,  for  all  the  phenomena 
connected  with  the  existence  of  animals  in  their  wild 
state  ....  Breeds  among  animals  are  the  work  of  man: 
Species  were  created  by  God."^ 

But  to  state  that  species  were  created  by  God  does  not 
satisfy  the  legitimate  curiosity  of  the  scientific  man. 
What  he  wishes  to  know  is:  By  what  method  was  creation 
accomplished?  God  might  have  worked  in  various  ways. 
Now,  the  study  of  Nature  has  never  revealed  to  us  but  one 
method  by  which  living  things  originate,  and  that  is  hy 
descent  from  preexisting  parents.  Agassiz's  hypothesis 
contradicts  this.  All  oaks  now-a-days  are  derived  by 
descent  from  preexisting  oaks,  but  the  first  oak,  accord- 
ing to  the  doctrine  of  special  creation,  was  created  by 
supernatural  means;  it  had  no  ancestors.  The  chief  objec- 
tion to  the  acceptance  of  this  hypothesis  is  that  the  more 
profoundly  and  accurately  we  study  living  things,  the  more 
obvious  it  becomes  that  truth  lies  in  another  direction. 

2.  Lamarck's  Hypothesis. — The  noted  French  naturaUst, 
Lamarck,  taught  that  all  living  things  have  been  derived 
from  preexisting  forms;  that  the  effects  of  use  and  disuse 
caused  changes  in  bodily  structure;  that  these  changes 
were  inherited  and  accentuated  from  generation  to  genera- 
tion; that,  being  of  use,  those  individuals  possessing  the 
changes  in  greatest  perfection  survived  while  others  per- 

*  Agassiz,  L.  "Methods  of  Study  in  Natural  History,"  Boston,  1893, 
pp.  146,  147. 


EVOLUTION  507 

ished;  and  that  the  derivation  of  new  species  is  thus 
accounted  for  in  a  simple  and  logical  manner.  By  con- 
tinual reaching  for  tender  leaves  on  high  branches,  the 
long  neck  of  the  giraffe  was  gradually  produced,  the  slight 
gain  in  length  in  one  generation  being  transmitted  by 
inheritance  to  the  next,  and  so  on. 

The  main  thesis  of  Lamarck,  as  stated  by  himself,  is 
as  follows: 

"In  animals  and  plants,  whenever  the  conditions  of 
habitat,  exposure,  climate,  nutrition,  mode  of  life,  et 
cetera,  are  modified,  the  characters  of  size,  shape,  relations 
between  parts,  coloration,  consistency,  and,  in  animals, 
agility  and  industry,  are  modified  proportionately." 

As  illustrating  the  direct  effect  of  environment  on  organ- 
isms, Lamarck  chose  a  plant,  the  water-buttercup  {Ran- 
unculus aquatilis),  which  may  grow  in  marshy  places,  or  im- 
mersed in  water.  When  immersed,  the  leaves  are  all  finely 
divided,  but  when  not  immersed,  they  are  merely  lobed. 

While  plants  are  more  passive,  and  are  affected  by  their 
surroundings  directly,  through  changes  in  nutrition,  light, 
gravity,  and  so  on,  animals  react  to  environmental  changes 
in  a  more  positive  and  less  passive  manner.  Thus,  in 
the  words  of  Lamarck:^ 

''Important  changes  in  conditions  bring  about  impor- 
tant changes  in  the  animals'  needs,  and  changes  in  their 
needs  bring  about  changes  in  their  actions.  If  the  new 
needs  become  constant  or  durable,  the  animals  acquire 
new  habits.  .  ,  .  Whenever  new  conditions,  becoming 
constant,  impart  new  habits,  to  a  race  of  animals  .  .  . 
these  habitual  actions  lead  to  the  use  of  a  certain  part  in 

^Translated  from  his  Philosophie  Zoologique,  vol.  I,  pp.  227,  223,  224, 
248. 


5o8  STRUCTURE    AND    LIFE    HISTORIES 

preference  to  another,  or  to  the  total  disuse  of  a  part  which 
is  now  useless.  .  .  .  The  lack  of  use  of  an  organ,  made 
constant  by  acquired  habits,  weakens  it  gradually  until 
it  degenerates  or  even  disappears  entirely."  Thus,  "it 
is  part  of  the  plan  of  organization  of  reptiles,  as  well  as  of 
other  vertebrates,  that  they  have  four  legs  attached  to 
their  skeleton  .  .  .  but  snakes  acquired  the  habit  of  glid- 
ing over  the  ground  and  concealing  themselves  in  the  grass; 
owing  to  their  repeated  efforts  to  elongate  themselves,  in 
order  to  pass  through  narrow  spaces,  their  bodies  have 
acquired  a  considerable  length,  not  commensurate  with 
their  width.  Under  the  circumstances,  legs  would  serve 
no  purpose  and,  consequently,  would  not  be  used,  long 
legs  would  interfere  with  the  snakes'  desire  for  gliding, 
and  short  ones  could  not  move  their  bodies,  for  they  can 
only  have  four  of  them.  Continued  lack  of  use  of  the 
legs  in  snakes  caused  them  to  disappear,  although  they 
were  really  included  in  the  plan  of  organization  of  those 
animals." 

On  the  other  hand,  ''the  frequent  use  of  an  organ,  made 
constant  by  habit,  increases  the  faculties  of  that  organ, 
develops  it  and  causes  it  to  acquire  a  size  and  strength  it 
does  not  possess  in  animals  which  exercise  less.  A  bird, 
driven  through  want  to  water,  to  find  the  prey  on  which 
it  feeds,  will  separate  its  toes  whenever  it  strikes  the  water 
or  wishes  to  displace  itself  on  its  surface.  The  skin  uniting 
the  bases  of  the  toes  acquires,  through  the  repeated  separ- 
ating of  the  toes,  the  habit  of  stretching;  and  in  this  way 
the  broad  membrane  between  the  toes  of  ducks  and  geese 
has  acquired  the  appearance  we  observe  to-day." 

If  such  modifications  are  acquired  by  both  sexes  they 
are  transmitted  by  heredity  from  generation  to  generation. 


EVOLUTION  509 

One  of  the  weaknesses  in  Lamarck's  hypothesis  appears 
in  his  illustration  of  the  snake.  If  we  should  grant  that 
inheritance  of  the  effects  of  disuse  of  the  legs  might  possi- 
bly explain  their  absence  in  snakes,  still  it  would  not  ex- 
plain the  origin  of  the  snake's  desire  to  glide.  That  is,  of 
course,  as  much  a  characteristic  of  the  snake  as  the  absence 
of  legs. 

Other  arguments  against  the  validity  of  Lamarckism 
are :  first,  that  no  one  has  ever  been  able  to  prove,  by  ex- 
periment or  otherwise,  that  the  effects  of  use  (the  so-called 


Fig.  388a. — Jean  Baptiste  Lamarck  (i 744-1829). 

"acquired  characters")  are  inheritable,  while  innumerable 
facts  indicate  that  they  are  not;  second,  the  hypothesis 
could  apply  only  to  the  animal  kingdom,  since  plants  in 
general  have  no  nervous  and  muscular  activities  like  those 
of  animals.  A  hypothesis  of  organic  evolution,  to  be  valid, 
must  apply  equally  to  both  plants  and  animals. 

3.  Darwin's  Hypothesis. — This  will  be  outlined  in  the 
next  chapter. 


CHAPTER  XXXII 
DARWINISM 

439.  Charles  Darwin. — The  question  of  the  method  of 
evolution  continued  to  be  debated,  with  no  satisfactory  solu- 
tion in  sight,  until  1859,^  when  Charles  Darwin  pubhshed 
the  greatest  book  of  the  nineteenth  century,  and  one  of  the 
greatest  in  the  world's  history,  the  Origin  of  Species.^ 
This  book  was  the  result  of  over  20  years  of  careful 
observation  and  thought.  It  consisted  of  the  elaboration 
of  two  principal  theories:  (i)  that  evolution  is  the  method 
of  creation;  (2)  that  natural  selection  is  the  method  of 
evolution. 

440.  Early  Antagonism  to  Evolution. — The  conception 
that  evolution  (as  distinguished  from  periodic,  super- 
natural interventions  of  the  Deity)  is  the  method  of 
creation  was  arrived  at  independently  by  Darwin,  but  was 
not  new  with  him.  As  we  have  just  seen,  it  was  proposed 
by  Lamarck.  Greek  philosophers  2,000  years  previously 
had  suggested  the  idea;  but  it  had  never  won  the  general 
acceptance  of  the  educated  world,  partly  because  it  was 
feared  to  be  anti-religious,  partly  because  it  was  never 
substantiated  by  sufficiently  convincing  evidence,  and 
partly  because  of  the  antagonism  of  a  few  men  of  great 

^  This  date  should  be  memorized.  It  is  one  of  the  most  important  in 
the  whole  history  of  human  thought. 

2  The  full  title  of  the  book  was,  "The  Origin  of  Species  by  Natural  Selec- 
tion, or  the  Preservation  of  Favored  Races  in  the  Struggle  for  Life." 

5 10 


DARWINISM 


511 


influence  in  the  world  of  intellect.     Men  preferred  to  fol- 
low a  leader,  more  or  less  blindly,  rather  than  take  the 


Fig.  389. — Charles  Darwin.  The  publication  of  his  "Origin  of  Species,'' 
in  1859,  revolutionized  human  thought,  and  gave  direction  to  all  scientific 
and  philosophic  thinking  from  that  time  to  the  present. 


pains  to  examine  the  voluminous  evidence  for  themselves, 
and  accept  the  logical  conclusion  without  prejudice  or 


512  STRUCTURE    AND    LIFE    HISTORIES 

fear,  wherever  it  might  lead  them,  or  however  much  it 
might  contradict  all  their  prejudice  and  preconceived 
notions.  But  truth  will  always,  in  the  end,  command 
recognition  and  acceptance,  and  there  is  almost  no  scien- 
tific man,  now-a-days,  who  does  not  regard  evolution  as 
axiomatic.  It  is  one  of  the  most  basic  of  all  conceptions, 
not  only  in  the  natural  and  the  physical  sciences,  but  also 
in  history,  sociology,  philosophy,  and  religion;  it  has,  in- 
deed, completely  revolutionized  every  department  of 
human  thought. 

441.  Darwinism. — It  is  the  second  of  the  above  men- 
tioned theories,  i.e.,  natural  selection,  that  constitutes  the 
essence  of  Darwinism.  The  theory  is  based  upon  five 
fundamental  facts,  which  are  matters  of  observation,  and 
may  be  verified  by  anyone,  as  follows: 

1.  Inheritance. — Characteristics  possessed  by  parents 
tend  to  reappear  in  the  next  or  in  succeeding  generations. 
We  are  all  familiar  with  the  fact  that  children  commonly 
resemble  one  or  both  parents  or  a  grandparent,  or  great 
grandparent  in  some  characteristic.  From  this  we  infer 
that  something  has  been  inherited  from  the  ancestor  which 
causes  resemblance  in  one  or  more  characters — physical  or 
mental. 

2.  Variation. — But  the  expression  of  the  inheritance  is 
seldom,  if  ever,  perfect.  Eyes  are  a  little  less  or  a  little 
more  brown;  stature  is  never  just  the  same;  one-half  the 
face  may  resemble  a  given  ancestor  more  than  another; 
petals  may  be  more  or  less  red  or  blue;  no  two  oranges 
taste  exactly  alike;  no  two  maple  leaves  are  of  precisely 
the  same  shape.  There  is  inheritance,  but  inheritance  is 
usually  expressed  with  modifications  or  variations  of  the 
ancestral  type. 


DARWINISM  513 

3.  Fitness  for  Environment. — It  is  common  knowledge 
that  living  things  must  be  adjusted  to  their  environment. 
Poor  adjustment  means  sickness  or  weakness;  complete 
or  nearly  complete  lack  of  adjustment  means  death. 
Water-lilies,  for  example,  cannot  live  in  the  desert, 
cacti  cannot  live  in  salt  marshes;  cocoanuts  cannot  be 
grown  except  in  subtropical  or  tropical  climates,  edelweiss 
will  not  grow  in  the  tropics.  This  is  because  these  various 
kinds  of  plants  are  so  organized  that  they  cannot  adjust 
themselves  to  external  conditions,  beyond  certain  more  or 
less  definite  limits  or  extremes.  A  cactus  is  fit  to  live  in 
the  desert  because  it  is  protected  by  its  structure  against 
excessive  loss  of  water,  and  has  special  provision  for 
storing  up  water  that  may  be  used  in  time  of  drought. 
Deciduous  trees  are  fitted  to  live  in  temperate  regions, 
partly  because  their  deciduous  habit,  and  their  formation 
of  scaly  buds  enables  them  to  withstand  the  drought  of 
winter.  Negroes  live  without  discomfort  under  the  trop- 
ical sun  because  they  are  protected  by  the  black  pigment 
in  their  skin.  And  so,  in  countless  ways,  we  might  illus- 
trate the  fact  that  all  living  things,  in  order  to  flourish, 
must  be  adjusted  to  their  surroundings. 

4.  Struggle  for  Existence. — The  clue  to  the  method  of 
evolution  first  dawned  upon  Darwin  in  1838,  while  reading 
Malthus  on  "Population."  Malthus  emphasized  the  fact 
that  the  number  of  human  beings  in  the  world  increased 
in  geometrical  ratio  (by  multiplication) ,  while  the  food  sup- 
ply increased  much  less  rapidly  by  arithmetical  ratio  (by 
addition).  Therefore,  argued  Malthus,  the  time  will  soon 
be  reached  when  there  will  not  be  food  enough  for  all; 
men  will  then  struggle  for  actual  existence,  and  only  the 
fittest  (i.e.,  the  strongest,  the  fleetest,  the  most  clever  or 


514  STRUCTURE   AND    LIFE    HISTORIES 

cunning)  will  survive.  In  pondering  this  hypothesis 
Darwin  at  once  saw  its  larger  application.^  There  are 
always  more  progeny  produced  by  a  plant  or  an  animal 
than  there  is  room  and  food  for,  should  they  all  survive. 
Darwin  showed  that  the  descendants  of  a  single  pair  of 
elephants  (one  of  the  slowest  breeders  of  all  animals) 
would,  if  all  that  were  born  survived,  reach  the  enormous 
number  of  19,000,000  in  from  740  to  750  years. ^  But 
the  total  number  of  elephants  in  the  world  does  not  appre- 
ciably increase :  evidently  many  must  perish  for  every  one 
that  lives.  There  must  therefore  be  an  intense  struggle 
for  existence.     Darwin^  gives  the  following  illustration: 

'' Seedlings,  also,  are  destroyed  in  vast  numbers  by 
various  enemies;  for  instance,  on  a  piece  of  ground  3 
feet  long  and  2  wide,  dug  and  cleared,  and  where  there 
could  be  no  choking  from  other  plants,  I  marked  all  the 
seedlings  of  our  native  weeds  as  they  came  up,  and  out  of 
357  no  less  than  295  were  destroyed,  chiefly  by  slugs  and 
insects.  If  turf  which  has  long  been  mown,  and  the  case 
would  be  the  same  with  turf  closely  browsed  by  quadru- 
peds, be  let  to  grow,  the  more  vigorous  plants  gradually 

^  "in  October  1838,"  says  Darwin,  "that  is,  15  months  after  I  had 
begun  my  systematic  inquiry,  I  happened  to  read  for  amusement  'Malthus 
on  Population,'  and  being  well  prepared  to  appreciate  the  struggle  for 
existence  which  everywhere  goes  on  from  long-continued  observation  of 
the  habits  of  animals  and  plants,  it  at  once  struck  me  that  under  these 
circumstances  favorable  variations  would  tend  to  be  preserved,  and 
unfavorable  ones  to  be  destroyed.  The  result  of  this  would  be  the  forma- 
tion of  new  species.  Here  then  I  had  at  last  got  a  theory  by  which  to 
work." 

2  One  pair  of  elephants  produces  an  average  of  only  one  baby  elephant 
in  10  years,  and  the  breeding  period  is  confined  to  from  about  the  30th  to 
the  90th  year.  For  illustrations  of  the  prolific  nature  of  plants,  see 
paragraph  173,  pp.   190-191. 

*  "Origin  of  Species"  (New  York,  1902  edition),  pp.  83,  84. 


DARWINISM  515 

kill  the  less  vigorous,  though  fully  grown  plants;  thus  out 
of  20  species  growing  on  a  little  plot  of  mown  turf  (3  feet 
by  4)  nine  species  perished,  from  the  other  species  being 
allowed  to  grow  up  freely.'' 

^^ Struggle  for  Existence'^  Used  in  a  Large  Sense. — "I 
should  premise,"  said  Darwin,  "that  I  use  this  term  in  a 
large  and  metaphorical  sense  including  dependence  of  one 
being  on  another,  and  including  (which  is  more  important) 
not  only  the  hfe  of  the  individual,  but  success  in  leaving 
progeny.  Two  canine  animals,  in  a  time  of  dearth,  may 
be  truly  said  to  struggle  with  each  other  which  shall  get 
food  and  live.  But  a  plant  on  the  edge  of  a  desert  is  said 
to  struggle  for  Hfe  against  the  drought,  though  more 
properly  it  should  be  said  to  be  dependent  on  the  moisture. 
A  plant  which  annually  produces  a  thousand  seeds,  of 
which  only  one  on  an  average  comes  to  maturity,  may  be 
more  truly  said  to  struggle  with  the  plants  of  the  same  and 
other  kinds  which  already  clothe  the  ground.  The  mistle- 
toe is  dependent  on  the  apple  and  a  few  other  trees,  but 
can  only  in  a  far-fetched  sense  be  said  to  struggle  with 
these  trees,  for,  if  too  many  of  these  parasites  grow  on  the 
same  tree,  it  languishes  and  dies.  But  several  seedling 
mistletoes,  growing  close  together  on  the  same  branch,  may 
more  truly  be  said  to  struggle  with  each  other.  As  the 
mistletoe  is  disseminated  by  birds,  its  existence  depends 
on  them;  and  it  may  metamorphically  be  said  to  struggle 
with  other  fruit-bearing  plants,  in  tempting  the  birds  to 
devour  and  thus  disseminate  its  seeds.  In  these  several 
senses,  which  pass  into  each  other,  I  use  for  convenience 
sake  the  general  term  of  Struggle  for  Existence." 

5.  Survival  of  the  Fittest. — In  this  struggle  for  existence 
only  those  best  suited  to  their  environment  will  survive. 


5l6  STRUCTURE    AND    LIFE    HISTORIES 

The  dandelion  from  the  seed  that  germinates  first  secures 
the  best  light;  the  one  that  sends  down  the  longest  and 
most  vigorous  root-system,  that  produces  the  largest,  most 
rapidly  growing  leaves  will  survive,  and  will  tend  to  trans- 
mit its  vigorous  qualities  to  its  progeny.  Less  vigorous 
or  less  ''lit"  individuals  perish.  To  this  phenomenon 
Herbert  Spencer  applied  the  phrase,  ''survival  of  the  fit- 
test." Darwin  called  it  ''natural  selection,"  because  it 
was  analogous  to  the  artificial  selection  of  favored  types 
by  breeders  of  plants  and  animals.  It  will  be  readily  seen, 
however,  that  the  process  in  nature  is  not  so  much  a  selec- 
tion of  the  fittest,  as  a  rejection  of  the  unfit;  the  unfit  are 
eliminated,  while  the  fit  survive.  It  has  been  suggested 
that  "natural  rejection"  would  be  a  better  name  than 
"natural  selection."  "Variations  neither  useful  nor  in- 
jurious," said  Darwin,  "would  not  be  affected  by  natural 
selection." 

442.  Difficulties  and  Objections.^The  publication  of 
Darwin's  "Origin  of  Species"  aroused  at  once  a  storm  of 
opposition.  Theologians  opposed  the  theory  because  they 
thought  it  eliminated  God.  Especially  bitter  antagonism 
was  aroused  by  Darwin's  suggestion  that,  by  means  of 
his  theory  "much  light  will  be  thrown  on  the  origin  of 
man  and  his  history."  The  unthinking  and  the  careless 
thinkers  accused  Darwin  of  teaching  that  man  is  descended 
from  monkeys.  Neither  of  these  accusations,  however, 
was  true.  Darwinism  neither  eliminates  God,  nor  does  it 
teach  that  monkeys  are  the  ancestors  of  men. 

By  slow  degrees,  however,  men  began  to  give  more  care- 
ful and  unprejudiced  attention  to  the  new  theory,  and  not 
to  pass  adverse  judgment  upon  it  until  they  were  sure  they 
understood  it.     "A   celebrated   author   and   divine   has 


DARWINISM  517 

written  to  me,"  says  Darwin,  ''that  he  has  gradually 
learnt  to  see  that  it  is  just  as  noble  a  conception  of  the 
Deity  to  believe  that  He  created  a  few  original  forms  capa- 
ble of  self-development  into  other  and  needful  forms,  as 
to  believe  that  He  required  a  fresh  act  of  creation  to  supply 
the  voids  caused  by  the  action  of  His  laws." 

And  in  closing  his  epoch-making  book,  Darwin  called 
attention  to  the  fact  that,  in  the  light  of  evolution,  all 
phases  of  natural  science  possess  more  interest  and  more 
grandeur. 

''When  we  no  longer  look  at  an  organic  being  as  a  savage 
looks  at  a  ship,  as  something  wholly  beyond  his  compre- 
hension; when  we  regard  every  production  of  nature  as 
one  which  has  had  a  long  history;  when  we  contemplate 
every  complex  structure  and  instinct  as  the  summing  up 
of  many  contrivances,  each  useful  to  the  possessor,  in  the 
same  way  as  any  great  mechanical  invention  is  the  sum- 
ming up  of  the  labour,  the  experience,  the  reason,  and  even 
the  blunders  of  numerous  workmen;  when  we  thus  view 
each  organic  being,  how  far  more  interesting — I  speak  from 
experience — does  the  study  of  natural  history  become!" 

"It  is  interesting  to  contemplate  a  tangled  bank,  clothed 
with  many  plants  of  many  kinds,  with  birds  singing  on  the 
bushes,  with  various  insects  flitting  about,  and  with  worms 
crawling  through  the  damp  earth,  and  to  reflect  that  these 
elaborately  constructed  forms,  so  different  from  each 
other,  and  dependent  upon  each  other  in  so  complex  a 
manner,  have  all  been  produced  by  laws  acting  around  us. 
These  laws,  taken  in  the  largest  sense,  being  Growth  with 
Reproduction;  Inheritance  which  is  almost  implied  by 
reproduction;  Variability  from  the  indirect  and  direct 
action  of  the  conditions  of  life,  and  from  use  and  disuse; 


5l8  STRUCTURE    AND    LIFE    HISTORIES 

a  Ratio  of  Increase  so  high  as  to  lead  to  a  Struggle  for  Life, 
and  as  a  consequence  to  Natural  Selection,  entailing  Diver- 
gence of  Character  and  the  Extinction  of  less-improved 
forms.  Thus,  from  the  war  of  nature,  from  famine  and 
death,  the  most  exalted  object  which  we  are  capable  of 
conceiving,  namely,  the  production  of  the  higher  animals, 
directly  follows.  There  is  grandeur  in  this  view  of  life, 
with  its  several  powers  having  been  originally  breathed 
by  the  Creator  into  a  few  forms  or  into  one;  and  that, 
whilst  this  planet  has  gone  cycling  on  according  to  the 
fixed  law  of  gravity,  from  so  simple  a  beginning  endless 
forms  most  beautiful  and  most  wonderful  have  been,  and 
are  being  evolved." 

443.  Objections  from  Scientists. — Objections  to  Dar- 
win's theory  were  also  brought  forward  by  scientific  men — 
partly  from  prejudice,  but  chiefly  because  they  demanded 
(and  rightly)  more  evidence,  especially  on  certain  points 
which  seemed  at  variance  with  the  theory.  For  example, 
they  said,  no  one  has  ever  observed  a  new  species  develop 
from  another;  this  ought  to  be  possible  if  evolution  by 
natural  selection  is  now  in  progress.  The  absence  of 
"connecting  links,"  or  transitional  forms  between  two 
related  species  was  noted;  the  presence  of  apparently 
useless  characters  (of  which  there  are  plenty  in  both  ani- 
mals and  plants)  was  not  accounted  for;  and  the  geologists 
and  astronomers  claimed  that  the  time  required  for  evolu- 
tion to  produce  the  organic  world  as  we  now  behold  it  is 
longer  than  the  age  of  the  earth  as  understood  from  geolog- 
ical and  astronomical  evidence. 

There  is  not  space  here  to  summarize  the  answers  to  all 
these  objections.  Suffice  it  to  say  that  scientific  investi- 
gation since  Darwin's  time  has  given  us  reasonably  satis- 


DARWINISM  519 

factory  answers  to  most  of  them,  so  that  now  practically 
no  scientific  man  doubts  the  essential  truth  of  evolution; 
it  is  the  corner  stone  of  all  recent  science,  the  foundation 
of  all  modern  thought, 

444.  The  Modem  Problem. — But  Darwinism  left  us 
with  a  very  large  and  very  fundamental  problem  unsolved. 
Upon  what  materials  does  natural  selection  act  in  the 
formation  of  species?  Obviously  the  ''fittest"  survives, 
but  what  is  the  origin  of  the  fittest?  This  problem  Darwin- 
ism did  not  solve.  The  solution  of  it  is  one  of  the  most 
fundamental  and  important  tasks  now  being  undertaken 
by  biologists.  The  most  effective  attack  is  by  the  method 
of  experimental  evolution,  which  forms  the  subject  of  the 
next  chapter. 


CHAPTER  XXXIII 
EXPERIMENTAL  EVOLUTION 

445.  A  New  Method  of  Study. — Previous  to  Darwin's 
time  the  study  of  plants  and  animals,  was  carried  on 
chiefly  by  observations  in  the  field.  The  science  was 
largely  descriptive — a  record  of  what  men  had  observed 
under  conditions  over  which  they  did  not  endeavor  to 
exercise  any  control;  it  was  accurately  named  ''Natural 
History" — a  description  of  Nature.  But  Darwin  and  a 
few  of  his  contemporaries,  especially  among  botanists,  be- 
gan to  make  observations  under  conditions  which  they 
determined  and  largely  regulated.  In  this  way  the 
problems  were  simplified,  observation  became  more  ac- 
curate, and  the  endeavor  was  made  to  assign  the  prob- 
able causes  of  the  observed  phenomena.  With  the  intro- 
duction of  this  experimental  method,  science  began  to  make 
rapid  strides,  and,  more  than  ever  before,  facts  began  to 
be,  not  only  recorded,  but  interpreted  and  explained. 

446.  Hugo  de  Vries. — The  director  of  the  Botanic  Gar- 
den in  Amsterdam,  Holland,  Hugo  de  Vries,  was  among 
the  first  to  demonstrate  that  the  method  of  experiment 
may  be  applied  to  the  study  of  evolution.  His  plan  was 
to  secure  seed  of  a  given  species  from  a  plant  which  he 
believed  to  be  pure  with  reference  to  a  given  character, 
that  is,  not  contaminated  or  mixed  by  being  cross-pollin- 
ated with  another  variety  or  species.  The  characters  of 
the  parent  plant  were  carefully  noted  and  recorded  by 

520 


J 


EXPERIMENTAL   EVOLUTION 


521 


photographs  and  written  descriptions,  and  by  preserving 
dried  and  pressed  herbarium  specimens.  The  plants  of 
the  second  generation  were  carefully  guarded  from  being 
cross-pollinated,  and  thus  ''pure"  seed  were  secured  for  a 
third  generation.  This  was  continued  often  for  25  or  30 
generations  of  the  plant,  requiring  as  many  years  when  a 


Fig.  390, — Hugo  de  Vries.  His  pioneer  studies  of  osmosis  resulted  in 
fundamental  contributions  to  our  knowledge  of  that  subject;  his  mutation- 
theory  is  one  of  the  most  important  contributions  to  the  study  of  evolu- 
tion since  Darwin. 


species  produced  only  one  crop  of  seed  a  year.  Very  care- 
ful records  and  preserved  specimens  were  kept  of  the  plants 
of  each  generation,  and  accurate  comparisons  were  made 
to  see  if  any  individuals  showed  a  tendency  to  vary  widely 
from  their  parents  in  any  significant  way,  such  as  showing 
entirely  new  characters,  not  expressed  in  the  parents,  or 
failing  to  manifest  one  or  more  of  the  characters  of  the 
parent. 


52  2  STRUCTURE    AND    LIFE    HISTORIES 

447.  Two  Klinds  of  Variation. — One  of  the  first  results 
of  de  Vries's  painstaking  work  was  the  demonstration  of 
what  he  believed  to  be  a  fundamental  difference  between 
two  distinct  kinds  of  variation — continuous  (or  fluctuating) 
and  discontinuous  (or  saltative,  i.e.,  leaping). 

448.  Continuous  Variation. — Continuous  variation  is 
quantitative — a  case  merely  of  more  or  less.  It  deals  with 
averages.  Some  flowers  on  a  red-flowered  plant  may  be 
lighter  or  darker  red,  but,  in  a  series  of  generations,  the 
average  of  a  large  number  in  each  generation  does  not 
vary,  and  the  departure  from  the  average  never  exceeds 
certain  limits.  The  flowers  of  a  given  species  may  have  a 
certain  characteristic  odor,  but  the  odor  may  be  stronger 
in  some  flowers  than  in  others,  or  in  some  individual 
plants  than  in  others.  The  plants  grown  from  a  handful  of 
beans  of  the  same  variety  may  vary  in  height  within  limits , 
but  the  average  height  of  a  large  number  will  not  vary  in 
successive  generations,  and  will  be  characteristic  of  the 
species  or  variety.  In  other  words,  continuous  or  fluc- 
tuating variation  is  variation  about  a  mean.  It  may 
be  illustrated  by  the  bob  of  a  swinging  pendulum,  which 
continually  fluctuates  within  definite  limits  about  the 
mean  position  assumed  when  the  pendulum  is  at  rest 
(Fig.  396). 

All  plants  and  animals  manifest  fluctuating  variation 
in  all  their  characters  (Fig.  391),  and  such  variations  are 
largely,  if  not  entirely,  dependent  upon  the  environment. 
A  slight  change  in  the  kind  of  food  elements  supplied,  or  in 
the  amount  of  water  or  sunlight  available  will  make  the 
leaves  or  petals  a  deeper  or  a  paler  color.  Rich  soil, 
favoring  a  more  abundant  food  supply,  will  cause  a  greater 
average  growth  than  poor  soil,  but  unless  the  seed  for 


EXPERIMENTAL   EVOLUTION  523 

future  generations  is  selected  from  the  tallest  plants, 
and  the  richness  of  the  soil  is  maintained,  the  plants  will 
revert  to  their  normal,  lower  average  of  height.  In  other 
words,  the  average  height  of  the  plants  of  any  given  variety 
is  a  constant  (unvarying)  character,  except  that  it  may  be 


Fig.  391. —Branch  oi  Brachychiton  diver sijolmm,  illustrating  fluctuating 
or  continuous  variation  in  the  shape  of  the  leaves  on  one  plant. 


temporarily  altered  by  careful  selection  of  seeds  from  the 
tallest  or  shortest  individuals,  or  by  choosing  the  largest 
or  the  smallest  seeds  from  any  given  plant,  or  by  making 
the  soil  richer  or  poorer.  When  the  selection  ceases,  and 
the  soil  is  maintained  at  average  fertility,  the  characteristic 
average  height  of  the  plants  is  restored. 


524  STRUCTURE    AND    LIFE   HISTORIES 

449.  Illustrations  of  Continuous  Variation. — In  a  quart 

of  beans,  for  example,  there  are  no  two  seeds  of  precisely 
the  same  proportion  or  size;  some  are  longer,  some  shorter. 
De  Vries  describes^  an  experiment  in  which  about  450 
beans  were  chosen  from  a  quantity  purchased  in  the  mar- 
ket, and  the  lengths  of  the  individuals  measured.  The 
length  varied  from  8  to  16  millimeters,  and  in  the  following 
proportions: 

Millimeters 8     9     lo       11        12        13     14     15     16 

Number  of  beans ...  .    i     2     23     108     167     106     33       7       i 

The  beans  were  then  placed  in  a  glass  jar  divided  into  nine 
compartments,  all  the  beans  of  the  same  length  in  the 
same  compartment.  When  this  was  done  it  was  found 
that  the  beans  were  so  grouped  that  the  tops  of  the  columns 
in  the  various  compartments  followed  a  curve,  known  as 
Quetelet's^  curve  (Fig.  392). 

This  curve  may  be  plotted  by  erecting  vertical  lines 
(ordinates)  at  intervals  of  i  millimeter  on  a  horizontal  line 
or  base,  the  height  of  each  vertical  line  being  proportionate 
to  the  number  of  beans  having  the  length  indicated  in 
figures  at  its  base.  This  curve  shows  the  frequency  of 
occurrence  of  seeds  of  any  given  dimension  between  the 
two  limits  or  extremes,  and  is  therefore  often  referred  to  as 
a  rMrve  of  frequency.  It  should  be  noted  that,  in  the  case 
illustrated,  the  greatest  frequency  (indicated  by  the  high- 
est point  of  the  curve)  very  nearly  coincides  with  the  aver- 
age dimension ;  in  other  words,  the  more  any  given  character 

^  De  Vries.     "The  Mutation  Theory,"  vol.  2,  p.  47,  Chicago,  1909. 

^  So  named  from  its  discoverer,  Qu6telet  (Ket-lay).  As  de  Vries 
states:  "For  a  more  exact  demonstration  a  correction  would  be  necessary, 
since  obviously  the  larger  beans  fill  up  their  compartment  more  than  a 
similar  number  of  small  ones." 


EXPERIMENTAL   EVOLUTION 


525 


departs  from  the  average  for 
that  character,  the  less  fre- 
quent is  its  occurrence. 

In  another  experiment, 
ears  of  corn,  harvested  from 
the  same  crop,  were  meas- 
ured and  found  to  vary 
in  length  from  4}^  inches 
to  9  inches;  the  largest  num- 
ber of  ears  (20)  were  7 
inches  long.  The  greater 
the  departure  from  this 
length,  in  either  direction, 
the  fewer  the  individuals; 
for  the  lengths  4  inches  and 

Fig.  392. — Demonstration  of 
Quetelet's  law  of  fluctuating  varia- 
bility in  the  length  of  seeds  of  the 
common  bean  (Phaseolus  vulgaris). 
Description  in  the  text.  (Redrawn 
from  de  Vries.) 


Fig.  393. — Curve  of  fluctuating  variation  (Quetelet's  curve),  formed  by- 
arranging  82  ears  of  corn  in  ten  piles,  according  to  the  length  of  the  ears. 
The  extremes  were  4.5  and  9  inches.  The  ears  were  taken  from  unselected 
material  from  a  field  of  corn.     (After  Blakeslee.) 


526  STRUCTURE    AND    LIFE   HISTORIES 


pt    J 

9 

Fig.  394. — Photograph  of  beans  rolling  down  an  inclined  plane  and 
accumulating  at  the  base  in  compartments,  which  are  closed  in  front  by 
glass.  The  exposure  was  long  enough  to  cause  the  moving  beans  to  appear 
as  caterpillar-like  objects  hopping  along  the  board.  If  we  assume  that 
the  irregularity  of  shape  of  the  beans  is  such  that  each  may  make  jumps 
either  toward  the  right  or  toward  the  left  in  rolling  down  the  board,  the 
laws  of  chance  lead  us  to  expect  that  in  very  few  cases  will  these  jumps 
be  all  in  the  same  direction,  as  indicated  by  the  few  beans  collected  in  the 
compartments  at  the  extreme  right  and  left.  Rather  the  beans  will  tend 
to  jump  in  both  right  and  left  directions,  the  most  probable  condition 
being  that  in  which  the  beans  make  an  equal  number  of  jumps  to  the  right 
and  to  the  left,  as  shown  by  the  large  number  accumulated  in  the  central 
compartment.  If  the  board  be  tilted  to  one  side,  the  curve  of  beans  would 
be  altered  by  this  one-sided  influence.  In  like  fashion,  a  series  of  factors — 
either  of  environment  or  of  heredity — if  acting  equally  in  both  favorable 
and  unfavorable  directions,  will  cause  a  collection  of  ears  of  corn  to  assume 
a  similar  variability  curve,  when  classified  according  to  their  relative  size. 
Such  curves,  called  Quetelet's  curves,  are  used  by  biometricians  in  classify- 
ing and  studying  variations  in  plants  and  animals.  (Photo  by  A.  F. 
Blakeslee.  Legend  slightly  modified  from  Journal  of  Heredity,  Tane, 
1916.) 


EXPERIMENTAL    EVOLUTION  527 

9  inches  the  frequency  was  zero.  When  the  ears  were 
arranged  in  piles  according  to  their  length,  the  tops  of 
the  piles  indicated  the  curve  of  frequency  (Fig.  393). 

The  curve  of  frequency  indicates  the  quantitative  dis- 
tribution of  any  character  or  quality  when  its  occurrence 
is  dependent  largely  upon  chance.  This  is  strikingly 
illustrated  by  the  grouping  of  bean  seeds  rolled  down  a 
smooth  inclined  plane,  and  collected  in  receptacles  at  the 
bottom  (Fig.  394).  The  seeds  are  started  rolling  midway 
between  the  edges  of  the  plane;  the  chances  are  about 
equal  for  some  of  the  seeds  to  fall  into  the  outside  compart- 
ments, but  the  odds  are  vastly  in  favor  of  their  landing  at 
or  near  the  center.  Thus  they  group  themselves  so  that 
the  tops  of  the  piles  form  a  curve  of  chance  variation. 
When  the  result  is  influenced  in  one  direction  more  than 
in  another  the  crest  of  the  curve  will  be  nearer  one  extreme 
than  the  other,  and  the  curve  is  to  that  extent  skew.  The 
curve  of  bean  seeds  in  Fig.  394  is  slightly  skew  toward 
the  right-hand  extreme.    Suggest  one  or  more  reasons  why. 

450.  Fluctuating  Variation  and  Inheritance. — When 
the  ancestry  is  not  mixed  or  hybrid  the  curve  of  frequency 
of  any  character  in  one  generation  ordinarily  tends  to 
recur  in  successive  generations  of  descendants,^  providing 
the  environment  remains  essentially  the  same. 

451.  Discontinuous  Variation. — Long  before  Darwin, 
students  of  plants  and  animals  had  observed  a  different 
kind  of  variation  than  continuous — one  which  was  not 
quantitative  but  qualitative,  resulting  in  the  expression  of 
new  characters,  or  of  a  new  curve  oj  frequency;  that  is,  in 
fluctuation  about  a  new  mean.     Plants  from  some  of  the 

^  The  behavior  of  hybrid  descendants  is  a  special  case  described  in 
::hapter  XXXVII. 


528  STRUCTURE    AND    LIFE    HISTORIES 

seeds  of  a  red-flowered  specimen  bear  flowers,  not  that 
vary  from  deeper  to  paler  red,  but  that  suddenly,  at  one  step, 
have  become  pure  white;  one  or  more  seeds  from  an  odor- 
less plant  may  give  rise  to  individuals  whose  flowers  are 
sweet-scented;  or  vice  versa,  odorless  specimens  may  spring 
at  one  leap,  not  by  gradual  minute  changes,  from  those  that 


Fig.  395. — Leaves  of  varieties  of  the  Boston  fern  (Nephrolepis),  showing 
(from  left  to  right)  progressive  branching  of  the  pinnae  and  pinnules,  and 
illustrating  so-called  "orthogenetic  saltation."     (After  R.  C.  Benedict.) 


are  fragrant;  in  one  generation  the  factors  controlling  height 
are  so  altered  that,  in  successive  generations,  the  average 
of  height  may  change  by  either  more  or  less,  so  that  the 
heights  of  the  individuals  fluctuate  about  a  new  mean.  In 
other  words,  we  recognize  a  second  type  of  variation — not 
the  fluctuation  of  individuals  about  an  unchanging  mean, 
but  the  appearance  of  a  new  mean,  about  which  the  given 
character  in  individuals  may  fluctuate. 


EXPERIMENTAL   EVOLUTION 


529 


When  discontinuous  variation  proceeds  along  a  definite 
line  through  several  successive  generations,  each  step  being 
an  intensification  of  the  preceding  one,  it  is  designated 
^'  or tho genetic  saltation^'  (^ig-  395)- 

461.  Illustration  of  the  Pendulum.— The  difference  be- 
tween discontinuous  and  fluctuating  variation  may  be 


V..,/.s,,^        Fluciuatin^ 
Extreme 


^■2^     F/uUuatn 
Extreme  c 

r-  Mean 


Extreme 


Meo-n 


Fig.  396. — Diagram  to  illustrate  the  difference  between  fluctuating 
variation  and  mutation;  0,  original  point  of  suspension;  M,  new  point  of 
suspension  after  the  mutation  has  occurred. 

aptly  illustrated  by  a  swinging  pendulum  (Fig.  396).     The 
vertical  position,  assumed  when  at  rest,  is  the  mean  of  all 
positions  that  may  be  assumed  as  the  pendulum  swings; 
34 


530  STRUCTURE    AND    LIFE    HISTORIES 

the  oscillation  about  this  mean  illustrates  continuous  or 
fluctuating  variation. 

But  we  may  conceive  that  the  point  of  suspension  of  the 
pendulum  changes,  as  shown  in  the  figure.  The  pendulum 
continues  to  oscillate,  but  now  about  a  new  mean  position; 
a  neiv  character  has  been  introduced,  with  its  own  fluctua- 
tions of  more  or  less. 

453.  Mutations. — Darwin,  as  well  as  others  before  and 
after  him,  recognized  both  kinds  of  variation,  but  de  Vries 
was  the  first  to  work  out  in  detail  the  hypothesis  that 
discontinuous  variations  furnish  the  material  for  natural 
selection.  Discontinuous  variations  he  called  mutations; 
plants  which  give  rise  to  or  ''throw"  them  are  said  to 
mutate.  A  plant  that  arises  by  mutation  is  an  elementary 
species,  or  mutant;  and  the  theory  that  mutations  (and  not 
fluctuations)  explain  the  origin  of  the  fittest,  and  supply 
the  materials  upon  which  natural  selection  operates  in  the 
formation  of  new  species,  de  Vries  called  the  ^nutation  theory. 

454.  Examples  of  Mutation. — The  kohlrabi,  cauli- 
flower, and  other  horticultural  varieties  of  the  wild  cliff- 
cabbage  (Fig.  397),  arc  believed  to  be  mutants,  and  to  have 
arisen,  not  by  the  prolonged  selection  of  fluctuating  varia- 
tions, but  at  one  step — in  one  generation — as  "sports" 
of  the  wild  Brassica  oleracea.  Strawberry  plants  without 
runners,  green  dahlias  and  green  roses,  the  common  seed- 
less bananas  of  the  markets,  the  Shirley  poppies,  pitcher- 
leaved  ash  trees,  Pierson's  variety  of  the  Boston  fern, 
5-9-  "leaved"  clovers  (Fig.  398),  white  black-birds  (and 
other  albinos,  including  albino  men),  moss-roses,  thornless 
cacti  and  thornless  honey-locusts,  red  sunflowers,  com- 
posites with  tubular  corollas  in  the  ray-flowers  (Fig.  399), 
and  the  innumerable  white  flowered  varieties  of  colored 


EXPERIMENTAL   EVOLUTION 


531 


532 


STRUCTURE    AND    LIFE    HISTORIES 


flowered  species,  are  all  illustrations  of  mutation.  Fre- 
quently the  mutative  change  occurs  in  a  lateral  bud,  pro- 
ducing a  ''bud-sport"  (Fig.  400).  Such  was  the  origin  of 
the  seedless  navel  orange  from  the  seed-bearing  orange. 


Fig.  398. — Clover  leaves  with  three  to  nine  leaflets,  illustrating  a 
tendency  to  mutate.  The  normal  clover  leaf  is  a  pinnately  compound 
leaf  with  three  leaflets.  Plants  with  leaves  having  five  to  nine  leaflets 
constitute  a  "half-race,"  i.e.  the  normal  character  is  active,  the  anomaly 
semi-latent.  (Photo  by  the  author;  specimens  from  cultures  of  G.  H. 
Shull.) 

455.  The  Evening-primrose. — In  1886  de  Vries  began 
to  search  for  a  species  that  was  in  a  mutating  condition, 
believing  that  any  given  species  is  at  some  periods  in  its 
history  more  labile  or  changeable  than  at  other  periods. 
After  a  long  search  he  found  in  an  abandoned  potato  field 
at  Hilversum,  near  Amsterdam,  a  large  number  of  plants 
of  Lamarck's  evening-primrose  {(Enothera  Lamarckiana) 
(Fig.  401.) 

"That  I  really  had  hit  upon  a  plant  in  a  mutable  period 
became  evident  from  the  discovery,  which  I  made  a  year 


EXPERIMENTAL   EVOLUTION 


533 


later,  of  two  perfectly  definite  forms  which  were  immedi- 
ately recognizable  as  two  new  elementary  species.  One 
of  them  was  a  short-styled  form:  O.  brevistylis,  which  at 
first  seemed  to  be  exclusively  male,  but  later  proved  to 
have  the  power,  at  least  in  the  case  of  several  individuals, 


Fig,  399. — Yellow  daisy,  or  cone-flower  {Rudbeckia  sp.),  showing  varia- 
tions of  the  character  of  mutations  in  the  ray-  and  disc-flowers.  At  d 
the  normally  ligulate  corollas  are  tubular;  at  /  they  have  all  aborted, 
except  two;  at  h  many  of  the  normally  tubular  disc-flowers  have  become 
ligulate,  making  a  nearly  "double  flower."     (Photo  by  E.  M.  Kittredge.) 

of  developing  small  capsules  with  a  few  fertile  seeds.  The 
other  was  a  smooth-leaved  form  with  much  prettier  foliage 
than  0.  Lamarckiana,  and  remarkable  for  the  fact  that 
some  of  its  petals  are  smaller  than  those  of  the  parent  type, 
and  lack  the  emarginate  form  which  gives  the  petals  of 


534 


STRUCTURE    AND    LIFE    HISTORIES 


Lamarckiana  their  cordate  character.     I   call   this  form 
O.  IcEvifolia." 

''When  I  first  discovered  them  (1887)  they  were  repre- 
sented by  very  few  individuals.  Moreover  each  form 
occupied  a  particular  spot  on  the  field.  O.  brevistylis 
occurred  quite  close  to  the  base  from  which  the  Oenothera 


Fig.  400. — A  plant  of  the  evening-primrose  {QLnolhera  biennis)  which, 
by  "bud  sporting,"  has  given  rise  (at  the  left)  to  a  branch  having  the 
characters  of  another  species. 

had  spread;  O.  IcBvifolia  on  the  other  hand,  in  a  small 
group  of  10  to  12  plants,  some  of  which  were  flowering 
whilst  others  consisted  only  of  radical  leaves,  in  a  part  of 
the  field  which  had  not  up  to  that  time  been  occupied  by 
0.  Lamarckiana.  The  impression  produced  was  that  all 
these  plants  had  come  from  the  seeds  of  a  single  mutant. 
Since  that  time,  both  the  new  forms  have  more  or  less 
spread  over  the  field"  {de  Vries), 


EXPERIMENTAL  EVOLUTION 


535 


Another  mutant  of  (E.  Lamarckiana  was  called  by 
de  Vries  (Enothera  gigas  (Fig.  402).  The  cell-nuclei  of 
this  mutant  have  twice  as  many  chromosomes  as  the 
parent  form. 


Fig.  401. — Lamarck's    evening-primrose     {(Enolhcra    Lanidrckiana).     A 
mutating  species.     (After  de  Vries.) 

456.  The  Test  of  a  Mutation.— The  deciding  test  as  to 
whether  a  given  new  form,  arising  without  crossing  from 
a  form  that  has  bred  true  for  at  least  two  generations,  is 
really  a  mutant  or  merely  a  fluctuating  variant,  is  to  see 
if  it  breeds  true  to  seed  for  the  new  character  or  characters. 
If  it  does  it  is  a  mutant;  otherwise  it  is  not.  It  is  clear, 
therefore,  that  the  only  way  the  problem  can  be  followed 
out  is  by  experiment — hence  the  term  experimental  evolu- 


536 


STRUCTURE    AND    LIFE    HISTORIES 


tion.  The  great  contribution  of  de  Vries  is  that  he  demon- 
strated that  evolution  may  be  studied  by  the  method  of 
experimentation.  The  next  step  for  him  to  take  after 
discovering  the  two  forms  that  he  supposed  to  be  mutants, 
was  to  breed  them  in  carefully  guarded,  pedigreed  cultures 


Fig.  402. — Giant  evening-primrose  QLnothera  gigas,  a  mutant  from 
(Enothera  Lamarckiana,  originated  in  1895.  (Cf.  Fig.  401.)  (After  de 
Vries.) 

in  his  garden,  and  also  to  breed  the  parent  form,  (Enothera 
Lamarckiana,  and  see  if  he  could  observe  the  two  forms 
above  mentioned,  or  other  mutants,  arise  from  seed  pro- 
duced without  crossing  with  any  other  species. 

The  entire  story  of  this  classical  series  of  experiments 
is  too  long  to  be  told  here.  Suffice  it  to  say  that  de  Vries 
did  observe  numerous  other  aberrant  forms  arise,  and  also 


EXPERIMENTAL    EVOLUTION  537 

found  that  they  bred  true  (except  for  additional  muta- 
tions) when  propagated  by  seed  for  over  25  years — that 
is,  they  were  true  mutations. 

467.  Relation  of  Mutation  Theory  to  Darwinism. — The 
mutation  theory  is  not  intended  by  de  Vries  to  supplant 
the  theory  of  natural  selection,  but  to  demonstrate  that 
the  materials  upon  which  selection  acts  in  the  formation 
of  new  species  are  mutations,  and  mutations  only — never 
fluctuating  or  individual  variations.  In  the  second  place 
the  mutation  theory  explains  away  numerous  objections 
to  natural  selection.  It  shows  how  characters  that  are 
never  of  vital  importance^ — i.e.,  matters  of  actual  life  or 
death — to  a  species  may  arise  and  be  perpetuated.  With- 
out mutation  this  is  difficult  to  explain,  ^  and  yet  many,  if 
not  most,  of  the  characteristics  by  which  different  species 
are  distinguished  from  each  other  are  of  this  kind — not, 
so  far  as  we  can  see,  absolutely  essential  to  the  life  of  the 
species.  Mutation  also  offers  a  method  by  which  evolu- 
tionary changes  may  take  place  within  a  much  shorter 
time- period  than  was  demanded  by  the  natural  selection 
of  fluctuations. 

Incidentally,  the  mutation  theory  clearly  shows  that  the 
absence  of  ''connecting  links"  between  species  is  no  argu- 
ment against  evolution,  but  is,  on  the  contrary,  just  what 
we  might  expect  to  find. 

458.  Value  of  the  Mutation  Theory.— As  stated  above, 
the  elaboration  of  the  mutation  theory  has  furnished  the 

^  As  required  by  Darwin's  theory.     See  quotation  on  p.  516. 

2  Other  explanations  have  been  offered.  For  example,  sometimes  two 
characters  appear  to  be  always  associated,  so  that  the  presence  of  one 
involves  the  presence  of  the  other;  as  a  mane  and  maleness  in  the  lion, 
dicotyledony  and  exogeny  in  Angiosperms. 


538  STRUCTURE    AND    LIFE    HISTORIES 

biological  world  with  a  new  method  of  study.  It  has 
demonstrated  that  the  method  of  evolution  may  be  studied 
by  experimentation,  and  this  demonstration  is,  probably, 
de  Vries's  greatest  service  to  science.  The  mutation 
theory  should  also  be  of  great  service  to  breeders.  It 
has  helped  to  establish  plant  and  animal  breeding  on  a 
more  scientific  basis,  has  pointed  the  way  to  correct 
methods  where  men  were  formerly  groping  in  the  dark, 
and  has  showed  that  results  of  commercial  value  do  not 
require  a  life  time,  but  may  be  obtained  within  two  or 
three  seasons.  By  the  application  of  modern  methods  it 
has  been  possible,  within  a  few  seasons,  to  obtain  new 
strains  of  oats  yielding  as  much  as  14  bushels  per  acre 
more  than  the  variety  from  which  they  were  derived,  and 
to  produce  new  strains  of  corn  not  only  giving  a  larger 
yield,  but  maturing  nearly  two  weeks  earlier  than  the 
parent  variety. 

459.  Classification. — Mere  information  is  not  science. 
A  ''book  of  facts"  is  not  a  scientific  treatise,  for  it  is 
composed  of  bits  of  unrelated  information,  presented  on 
some  artificial  basis  of  sequence,  as  for  example,  alpha- 
betically. Scientific  knowledge,  in  addition  to  being  as 
accurate  as  possible,  is  characterized  by  having  an  orderly 
arrangement  in  one's  mind,  and  this  order  is  based  on  a 
logical,  fundamental  relationship  between  the  facts  and 
ideas.  Only  by  such  an  arrangement  of  our  ideas  are  we 
able  to  understand  their  relation  to  each  other,  their  rela- 
tive importance,  and  their  real  significance.  Classifica- 
tion, therefore,  is  essential  to  all  science.  The  very 
existence  and  use  of  such  words  as  oaks,  maples,  roses, 
indicate  that  men  have  grouped  or  classified  their  ideas 
of  certain  plants  {e.g.,  red  oaks,  white  oaks,  black  oaks, 


EXPERIMENTAL    EVOLUTION 


539 


bur  oaks,  live  oaks,  etc.),  and  have  thereby  recognized 
that  certain  kinds  resemble  each  other  closely  enough  to  be 
placed  in  one  group  with  a  group-name.  All  the  com- 
mon names  of  plants  indicate  the  recognition  of  classes — 
a  classification. 


Fig.  403. — Linnaeus,  the  great  classifier  (1707-1778).  lie  is  wearing  a 
sprig  of  the  twin-flower  {Linncea  borealis),  one  of  his  favorite  flowers,  and 
named  after  him  by  his  friend,  Gronovius.  He  is  regarded  as  the  father 
of  modern  systematic  botany. 

460.  Evolution  and  Classification. — Without  the  guiding 
idea  of  evolution  classification  would  be  arbitrary  and 
artificial.  Linnaeus  classified  plants  on  the  basis  of  the 
number  of  stamens  they  possessed,  thus  placing  in  one 
group  plants  now  known  to  be  wholly  unrelated,  except 


540  STRUCTURE    AND    LIFE    HISTORIES 

that  they  have  a  chance  similarity  in  the  number  of 
stamens.  In  like  manner  we  may  group  together  plants 
with  red  flowers,  blue  flowers,  or  pink  flowers,  as  is  often 
done  in  "popular"  guides  to  the  wild  flowers.  This  has 
its  value,  but  it  tells  us  really  nothing  about  the  significant 
relationship  between  plants,  does  not  help  clear  up  our 
own  ideas,  does  not  show  the  gaps  in  our  knowledge  and 
tell  us  where  to  search  for  new  facts  to  fill  up  the  gaps. 
Evolution,  by  showing  that  plants  are  all  related  to  each 
other  by  descent,  just  as  are  the  members  of  a  large  family 
of  persons,  discloses  to  us  the  only  true  basis  of  classifica- 
tion— the  plan  that  endeavors  to  arrange  all  plants  so  as  to 
show  their  descent  and  their  relationship  to  each  other. 
Without  evolution  there  might  be  any  number  of  arbitrary 
systems;  on  the  basis  of  evolution  there  can,  in  the  end, 
be  but  one  true  system,  which  all  students  must  accept, 
because  it  will  be  a  true  record  of  what  has  actually  oc- 
curred in  the  history  of  development  of  the  plant  or  animal 
worlds.  In  other  words,  if  our  knowledge  should  ever  he- 
come  sufficiently  complete  and  exacts  the  classification  oj 
plants  would  give  a  summary — a  bird's  eye  view — of  the 
course  of  evolution  and  the  history  of  development.  To 
approximate  this  end  is  one  of  the  largest  problems  of  botany. 


CHAPTER  XXXIV 
HEREDITY 

461.  Importance  of  the  Study. — i.  To  Pure  Science. — 
No  knowledge  is  more  fundamental  than  a  correct  under- 
standing of  the  laws  of  heredity.  Its  fundamental  im- 
portance to  pure  science  becomes  evident  at  once  when  we 
consider  that,  since  evolution  has  been  accomplished  by 
the  descent  of  one  organism  from  another,  there  have  been 
one  or  more  unbroken  lines  of  inheritance  from  the  dawn 
of  plant  life  to  the  present.  Hence,  until  we  know  the 
laws  of  heredity,  we  cannot  fully  understand  expression, 
reproduction,  development,  variation,  sex,  or  evolution. 

2.  To  Applied  Science. — Correct  ideas  concerning  he- 
redity are  absolutely  essential  to  such  phases  of  applied 
science  as  animal  and  plant  breeding.  In  the  light  of  such 
knowledge  the  breeder  can  avoid  making  useless  experi- 
ments, and  can  accomplish  desired  results  more  quickly, 
more  cheaply,  and  with  greater  certainty  of  success. 

3.  To  Man. — A  correct  knowledge  of  the  principles  of 
heredity  is  vital  to  mankind ;  no  knowledge  is  more  so.  To 
realize  this,  we  have  only  to  reflect  that  our  own  characters 
are  very  largely  the  result  of  inheritance  from  our  ances- 
tors; and  not  only  our  characters,  but  our  physical  char- 
acteristics, our  vigor  of  mind  and  body,  our  capacity  for 
education,  our  susceptibility  to  disease,  and  often  the 
actual  existence  of  some  disease  within  our  bodies  or  minds. 

462.  Heredity  Reduced  to  Its  Lowest  Terms. — We  may 
study  heredity  under  the  very  simplest  conditions  in  the 

541 


542  STRUCTURE    AND    LIFE    HISTORIES 

descent  of  one-celled  organisms,  such  as  Pleurococcus. 
This  plant,  as  we  learned  in  Chapter  XVIII,  is  a  globule  of 
protoplasm,  containing  chlorophyll,  and  surrounded  by  a 
cellulose  cell-wall.  But  why  is  it  globular,  why  does  it 
contain  chlorophyll,  why  has  it  a  cell- wall  of  cellulose? 
Why  is  it  not  elliptical,  why  is  it  not  red  instead  of  green, 
why  does  it  have  a  cell-wall,  instead  of  existing  naked  like 
the  Plasmodium  of  a  slime-mold,  why  is  its  cell-wall  of 
cellulose,  rather  than  of  lignin  or  chitin? 

The  short  answer  is,  because  its  ancestors,  for  ages  and 
ages,  have  possessed  the  characteristics  which  now  char- 
acterize Pleurococcus  plants.  But  that  only  puts  the 
question  back  an  indefinite  number  of  generations.  The 
real  reason  is,  because  the  Pleurococcus  protoplasm  pos- 
sesses a  physical  and  chemical  constitution — or  in  other 
words  a  mechanism — that,  under  normal  external  condi- 
tions, manufactures  green  pigment  instead  of  red,  cellulose 
instead  of  lignin,  or  any  other  substance,  at  the  surface, 
and  makes  the  cell-wall  of  even  resistance  to  the  osmotic 
pressure  within,  thus  producing  a  sphere  and  not  an  ellip- 
soid, or  filament,  or  any  other  shape. 

463.  What  is  Inheritance? — When  the  Pleurococcus  cell 
divides,  this  wonderful,  invisible  mechanism — the  certain 
definite  physical  and  chemical  constitution — is  transmitted 
to  each  of  the  daughter-cells;  each,  in  other  words,  re- 
ceives Pleurococcus  protoplasm.  This  protoplasm,  with 
its  definite  organization,  constitutes  the  inheritance.  The 
daughter-cells  do  not  inherit  a  spherical  shape  (as  is  evident 
from  Fig.  183),  but  a  definite  kind  of  protoplasm,  cell-sap 
of  certain  osmotic  properties,  and  surface  cellulose  of  even 
elasticity,  so  that,  in  surroundings  uniform  on  all  sides, 
a  spherical  shape  must  finally  result.     The  shape  is  an 


HEREDITY  543 

expression  of  the  inheritance  for  the  given  environment. 
Under  different  external  conditions  the  expression  might 
he  different;  hut  the  inheritance  would  he  the  same.  The 
chlorophyll  in  the  daughter-cells,  immediately  after  cell- 
division,  is  a  direct  inheritance,  but  the  chlorophyll  subse- 
quently manufactured,  and  the  green  color  which  it  gives 
to  the  plant,  are  not  inherited;  they  are  expressions  of  the 
inheritance — which  in  this  instance  is  a  chloroplastid  that 
reproduces  itself  by  division,  and  manufactures  chlorophyll 
in  the  presence  of  sunlight.  Under  abnormal  external 
conditions  the  mechanism  may  not  act,  or  may  act  ab- 
normally, so  that  yellow  pigment  appears  instead  of  green 
— or  in  darkness  no  pigment  at  all.  In  either  case  the  in- 
heritance is  the  same,  but  the  expression  varies.  A  mod- 
ern writer  has  defined  inheritance  as  all  that  an  organism 
has  to  start  with.  It  is  the  protoplasmic  substance,  with 
all  its  potentialities,  passed  on  from  parent  to  offspring. 

464.  Inheritance  Versus  Expression. — In  the  light  of 
this  information,  obtained  by  a  study  of  the  lowly  Pleuro- 
coccus,  we  are  able  to  understand  that  what  we  inherit 
from  our  parents  or  grandparents,  is  not  a  certain  shape  of 
nose,  a  certain  characteristic  gait,  a  musical  or  mathe- 
matical bent  of  mind,  a  quick  temper,  but  a  substance 
(protoplasm)  possessing  a  very  delicate,  intricate,  and 
characteristic  constitution  or  mechanism.  Under  certain 
conditions  this  inheritance  may  so  express  itself  as  to 
cause  resemblance  in  some  physical  or  mental  trait;  or  it 
may  find  a  quite  different  expression,  as  when  parents  of 
medium  height  have  tall  children,  or  parents  musically 
inclined  have  children  that  do  not  care  for  music ;  or  sweet- 
peas  having  white  flowers  only,  produce,  when  crossed, 
peas  having  colored  flowers.     Or  again,  not  all  that  is  in- 


544  STRUCTURE    AND    LIFE    HISTORIES 

herited  may  be  expressed;  this  is  illustrated  when  chil- 
dren resemble,  not  their  parents,  but  their  grandparents. 
Here  the  parents  transmitted  an  inheritance  which,  in 
them,  found  no  expression.  A  remarkable  illustration  of 
inheritance  without  expression  is  seen  in  the  case  of  the 
alternation  of  generations  (pages  1 81-183)  The  initial 
protoplasm  of  the  sporophyte  is  all  inherited  through  the 
fertilized  egg  from  the  gametophytes,  but  most  of  the 
gametophytic  characters  do  not  appear  in  the  sporophyte, 
nor  do  the  typically  sporophytic  characters  find  expres- 
sion in  the  gametophyte.^ 

465.  Inheritance  Versus  Heredity. — As  stated  above, 
the  inheritance  is  that  which  is  actually  transmitted  from 
parent  to  offspring.  The  fern-spore,  for  example,  is  the 
inheritance  of  the  fern  gametophyte  from  the  sporophyte. 
Heredity  is  the  genetic  relationship  that  exists  between  suc- 
cessive generations  of  organisms.  The  relation  between  two 
brothers  and  their  parents  is  similar — it  is  one  of  heredity ; 
their  inheritance  may  be  quite  different. 

466.  Inheritance  and  Reproduction. — Inheritance  is,  of 
course,  inseparably  linked  with  reproduction  and  may  be 
studied  in  connection  with  the  three  following  types: 

I.  In  vegetative  propagation"^  the  new  plant,  as  noted  in 
Chapter  XVII,  is  obviously  only  a  portion  of  the  vegeta- 
tive tissue  of  the  parent  plant,  isolated  and  growing  inde- 
pendently by  itself.  The  separation  of  the  propagating 
piece  from  the  parent  is  often  (though  not  always)  mechan- 

^  The  chlorophyll,  of  course,  is  an  exception.  But  the  osmotic  strength 
of  the  cell-sap  is  a  different  expression  in  gametophyte  and  sporophyte, 
otherwise  the  young  sporophyte  could  not  live  parasitically  upon  the 
gametophyte. 

*  E.g.,  by  means  of  tubers,  cuttings  and  "slips,"  bulbs  and  bulbils, 
gemmae,  "runners,"  scions,  etc. 


HEREDITY 


545 


ical  and  artificial.  The  protoplasm  remains  unaltered  by 
the  act  of  separation,  reduction  divisions  are  not  involved, 
and  the  inheritance,  except  in  bud-variations,  is  unaffected 
by  the  change.     This  is  evident  in  those  cases  where  the 


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-<^. 

a 

1 

Fig.  404.^ — -Graft  of  tomato  {Lycopersicum  esculcnlum)  on  tobacco 
{Nicoiiana  tabacum).  On  the  tomato  are  grafted  Solanum  nigrum,  S. 
integrifolium,  and  Physalis  alkekengi.  Cf.  Fig.  243.  (Graft  made  by 
Mr.  M.  Free.) 

isolated  piece  is  grafted  upon  another  plant;  the  specific 
or  varietal  characteristics  of  the  scion  are  seldom,  if  ever, 
affected  by  the  stock.  Thus,  in  the  experiment  illustrated 
in  Fig.  404,  a  tomato  stem  was  grafted  upon  a  tobacco 

35 


546  STRUCTURE    AND    LIFE    HISTORIES 

plant,  and  upon  the  tomato  were  grafted  three  other 
species — Solanum  nigrum,  Solanum  integrifolium,  and 
Physalis  alkekengi.  Each  species  was  apparently  not  in 
the  least  altered  by  this  drastic  change  in  the  conditions 
of  its  life. 

2.  In  asexual  reproduction  hy  spores  the  situation  is 
quite  similar  to  that  in  vegetative  propagation,  but  in 
certain  cases  there  is  abundant  opportunity  for  the  proto- 
plasm to  become  more  or  less  altered  during  the  compli- 
cated changes  that  accompany  nuclear  division.  This  is 
especially  the  case  in  the  reduction  divisions  preceding 
spore-formation  in  the  sporophytes  of  higher  plants,  espe- 
cially when  the  plant  is  a  hybrid;  and  in  spore-formation 
in  the  sporangia  produced  from  the  zygospore  of  some  of 
the  filamentous  fungi,  like  Mucor  Mucedo.  In  the  latter 
case  the  nuclear  divisions,  some  time  preceding  spore-pro- 
duction, result  in  separating  out  the  female  (+)  and  male 
(— )  strains,  so  that  the  spores  in  a  given  sporangium  are 
unlike  as  to  sex — some  being  female  (+),  some  male  (— ). 
This  will  be  discussed  more  fully  in  the  next  chapter. 
Such  changes  result  merely  in  distributing  the  heritable 
units  {genes)  of  the  mother-cell  unequally  to  the  daughter- 
cells,  but  introducing  nothing  new;  they  may,  however, 
result  in  the  complete  loss  of  one  or  more  heritable  units, 
or  in  the  formation  of  a  new  one,  not  existent  in  the  parent. 
In  the  latter  two  cases  we  recognize  a  mutation.  No  hard 
and  fast  line  can  be  drawn  between  the  various  kinds  of 
asexual  reproduction;  there  are  various  degrees  of  transi- 
tion between  reproduction  by  spores,  gemmae,  bulbs  and 
tubers,  and  the  artificially  severed  buds  and  scions  used 
in  grafting  and  "slipping." 

3.  In  sexual  reproduction  there  intervene  between  par- 


HEREDITY  547 

ents  and  offspring,  not  only  the  complicated  reduction 
divisions  involved  in  the  formation  of  the  gametes,  but 
also  the  nuclear  and  cell-fusions  accomplished  by  the  union 
of  the  egg  and  sperm  in  fertilization.  Both  processes — 
the  formation  of  the  gametes,  and  their  fusion — offer 
almost  unlimited  opportunities  for  alterations  of  the  pro- 
toplasm— especially  that  of  the  nucleus.  This  method  of 
reproduction,  therefore,  has  the  very  greatest  interest  and 
importance  for  the  study  of  heredity.  In  the  fertilized 
egg^  are  united  the  inheritances  from  two  parents — from 
two  distinct  lines  of  ancestry — protoplasms  (germ-plasms) 
with  two  entirely  different  histories  extending  back  into 
the  past,  no  one  knows  how  far.  How  will  these  two 
inheritances  affect  each  other  when  they  intermingle  in 
the  fertilized  egg?  Will  one  tend  to  inhibit  or  check  cer- 
tain characteristics  or  functions  of  the  other;  will  they 
evenly  blend,  so  as  to  produce  an  expression  intermediate 
between  that  of  the  parents;  or  may  entirely  new  sub- 
stances be  formed  or  new  combinations  take  place,  result- 
ing in  an  entirely  new  expression  in  the  offspring? 

467.  Methods  of  Study. — To  endeavor  to  answer  the 
questions  just  asked  is  as  fascinating  an  occupation  as  it  is 
important,  and  the  answers  are  significant  for  man,  as  well 
as  for  plants.  It  is  indeed,  a  fortunate  thing  that  prin- 
ciples ascertained  by  studying  plants  apply  equally  to  man 
and  other  animals,  since  plants  are  so  much  easier  to 
handle  in  experimental  investigations. 

We  may  go  about  the  answering  of  these  questions  in 
either  of  two  ways.  We  may  gather  large  numbers  of 
statistics  to  measure  and  analyze  {statistical  or  hiometrical 

*  The  fertilized  egg  (as  Thomson  has  pointed  out)  is  the  inheritance, 
and  becomes,  in  the  mature  individual,  the  inheritor. 


548  STRUCTURE   AND    LIFE    HISTORIES 

method),  or  we  may  employ  the  experimental  method.  The 
method  of  biometry  enables  us  to  deal  with  a  larger  number 
of  individuals,  but  the  material  studied  is  usually  a  mixed 
population,  whose  history  is  only  imperfectly  known,  the 
conditions  are  more  complex,  and  little  if  at  all  under 
control.  By  the  experimental  method  it  is  not  necessary 
to  deal  with  such  large  numbers;  we  may  choose  carefully 
pedigreed  material  about  the  history  of  which  we  have 
more  or  less  accurate  knowledge,  and  we  may  greatly 
simplify  and  control  the  conditions  under  which  we  make 
our  observations.  The  largest  advance  toward  the  solu- 
tion of  the  problems  of  inheritance  has  been  made  by  the 
experimental  method,  in  the  form  first  employed  success- 
fully by  Gregor  Mendel.  This  method  will  be  briefly 
explained  in  the  next  chapter. 


CHAPTER  XXXV 

EXPERIMENTAL  STUDY  OF  HEREDITY 

468.  Gregor  Mendel.— Two   of   the   most   important 
contributions  ever  made  to  biological  science,   namely, 


_  Fig.  405.— Gregor  Mendel,  at  the  age  of  40.  His  theory  of  alternate 
inheritance  (Mendelism),  based  largely  on  experiments  with  the  garden 
pea,  IS  the  most  important  and  most  fruitful  contribution  ever  made  to 
the  study  of  inheritance. 

Mendel's  laws  of  heredity,  and  his  method  of  investigating 
them,  were  made  by  a  teacher  who  studied  plants  as  a  pas- 
time because  he  loved  to  do  it.     This  man  was  Gregor 

549 


550  STRUCTURE    AND    LIFE    HISTORIES 

Mendel,  a  monk  in  the  monastery  at  Briinn,  Austria,  where 
he  finally  became  abbott.  In  order  to  understand  his 
work  clearly  the  student  should  examine  carefully  the 
structure  of  the  edible  or  garden  pea,  the  chief  plant  with 
which  Mendel  worked. 

469.  MendePs  Problem. — It  was  a  favorite  study  of 
Mendel's  to  hybridize  {i.e.,  cross-pollinate)  plants  of  differ- 
ent species  and  varieties,  and  observe  the  behavior  of  the 
resulting  hybrids  in  successive  generations.  The  problem 
which  he  endeavored  to  solve  was  the  law  or  laws  "gov- 
erning the  formation  and  development  of  hybrids,"^  with 
special  reference  to  the  laws  according  to  which  various 
characters  of  parents  appear  in  their  offspring. 

470.  MendePs  Method. — He  recognized  that,  in  order 
to  solve  the  problem,  attention  must  be  given  to  at  least 
three  points,  as  follows: 

1.  "To  determine  the  number  of  different  forms  under 
which  the  offspring  of  hybrids  appear." 

2.  "To  arrange  these  forms  with  certainty  according  to 
their  generations." 

3.  "To  ascertain  accurately  their  statistical  relations," 
that  is,  to  express  the  results  quantitatively. 

No  previous  student  had  recognized  the  fundamental 
importance  of  these  requirements. 

471.  Choice  of  Material. — Mendel  realized  that  the 
success  of  any  experiment  depends  upon  choosing  the 
most  suitable  material  with  which  to  experiment.  He 
laid  down  the  requirements  as  follows: 

^  All  the  quotations  in  this  chapter  are  from  an  English  translation  of 
Mendel's  original  paper.  His  form  of  expression  has  been  preserved 
as  far  as  possible,  even  when  the  "quotes"  are  omitted. 


EXPERIMENTAL    STUDY    OF    HEREDITY  55 1 

1.  "The  experimental  plants  must  necessarily  possess 
constant  differentiating  characters.^^'^ 

2.  "The  hybrids  of  such  plants  must,  during  the  flower- 
ing period,  be  protected  from  the  influence  of  all  foreign 
pollen,  or  be  easily  capable  of  such  protection." 

3.  "The  hybrids  and  their  offspring  should  suffer  no 
marked  disturbance  in  their  fertility  in  the  successive 
generations." 

Mendel  also  called  attention  to  the  advantage  of  choos- 
ing plants  which,  like  the  peas,  are  easy  to  cultivate  in 
the  open  ground  and  in  pots,  and  which  have  a  relatively 
short  period  of  growth. 

472.  Characters  Chosen  for  Observation. — "Each  pair 
of  differentiating  characters  [have  been  thought  to]  unite 
in  the  hybrid  to  form  a  new  character,  which  in  the  pro- 
geny of  the  hybrid  is  usually  variable.  The  object  oj  the 
experiment  was  to  observe  these  variations  in  the  case  of  each 
pair  of  differentiating  characters,  and  to  deduce  the  law  ac- 
cording to  which  they  appear  in  successive  generations.  The 
experiment  resolves  itself  therefore  into  just  as  many 
separate  experiments  as  there  are  constantly  differentia- 
ting characters  presented  in  the  experimental  plants." 
The  following  were  the  characters  chosen  for  observation: 

I.  The  difference  in  the  shape  of  the  ripe  seeds  (round 
and  smooth  vs.  angular  and  wrinkled). 

^  Differentiating  characters  are  those  in  respect  to  which  the  two  species 
or  varieties  to  be  crossed  differ.  The  possession  of  chlorophyll  by  the  leaves 
of  peas,  for  example,  is  a  common  character.  "  Common  characters  are 
transmitted  unchanged  to  the  hybrids  and  their  progeny."  The  color  of 
the  corolla  (for  example,  white  in  one  species  and  purple  in  the  other)  is  a 
differentiating  character,  serving  to  differentiate  or  distinguish  one  species 
from  another. 


552 


STRUCTURE    AND    LIFE    HISTORIES 


2.  The  difference  in  the  color  of  the  cotyledons  (pale 
or  bright  yellow,  or  orange  vs.  light  or  dark  green). 

3.  The  difference  in  the  color  of  the  seed-coat  (white 
vs.  gray,  gray-brown,  leather- 
brown,  with  or  without  violet 
spotting,  etc.). 

4.  The  difference  in  the  form  of 
the  ripe  pods  (deeply  constricted 
between  the  seeds  and  more  or 
less  wrinkled,  or  the  opposite). 

5.  The  difference  in  the  color 
of  the  unripe  pods  (light  or  dark 
green  vs.  vivid  yellow) . 

6.  The  difference  in  the  posi- 
tion of  the  flowers  (i.e.j  axial  vs. 
terminal) . 

7.  The  difference  in  the  length 
of  the  stem  (the  extremes  chosen 
were  ''tails"  6  to  7  feet,  and 
''dwarfs"  %  feet  to  i}i  feet  in 
height). 

473.  Artificial  Hybridizing. — 
The  edible  pea  is  commonly  self- 
fertilized;  therefore,  to  make 
crosses  it  is  necessary  carefully  to 
remove  the  stamens  of  one  flower 
before  the  anthers  have  begun  to  shed  their  pollen,  and 
then  place  pollen  from  another  flower  on  the  stigma.  The 
flowers  must  then  be  carefully  guarded,  e.g.,  by  tying 
paper  bags  over  them  (Fig.  406),  to  prevent  other  pollen 
being  deposited  by  insects  or  otherwise.  In  this  way  the 
experimenter  knows  just  what  characteristics  enter  into 


Fig.  406. — Method  of  pro- 
tecting flowers  from  foreign 
pollen  by  paper  bags,  in  plant- 
breeding  experiments.  (After 
O.  E.  White.) 


EXPERIMENTAL    STUDY    OF   HEREDITY  553 

the  hybrid.  Careful  record  is  kept  of  all  data,  and  plants 
produced  in  this  way,  with  ancestral  characters  noted 
and  recorded,  are  called  pedigreed.  Plantings  of  such 
plants  are  called  pedigreed  cultures. 

In  many  species,  in  "making  the  cross '^  {i.e.,  doing  the 
cross-pollinating)  great  care  must  be  taken  to  avoid  con- 
tamination from  foreign  pollen,  of  which  the  air  may  be 
full.^  The  fingers  and  all  instruments  are  usually  rinsed 
in  alcohol  before  each  operation,  to  insure  kilUng  any 
foreign  pollen  that  might  be  present.  Numerous  other 
precautions  are  also  taken. 

When  the  hybrid  plants  are  mature,  careful  observa- 
tions of  whatever  character  is  under  observation  are 
made  and  recorded.  Whenever  possible  the  observation 
should  be  quantitative. 

474.  MendePs  Discoveries. — We  may  illustrate  Men- 
del's results  in  a  simple  manner  by  choosing,  as  the 
pair  of  contrasted  characters,  smooth  and  wrinkled  seeds 
of  the  pea.  Removing  all  the  stamens  from  flowers  of  a 
variety  having  smooth  seeds,  he  pollinated  those  flowers 
with  pollen  from  a  plant  bearing  wrinkled  seeds. 

It  should  now  be  kept  clearly  in  mind  just  what  the 
inheritance  of  the  fertilized  egg  is  in  such  a  case.  From 
the  pistillate  plant  the  inheritance,  contributed  by  the 
egg-cell,  included  the  protoplasmic  properties  (whatever 
they  may  be)  which,  when  free  to  produce  their  effect, 
cause  smooth  seeds;  from  the  staminate  parent  the  in- 
heritance, contributed  by  the  sperm-cell,  included  the 
protoplasmic  properties,  which,  when  free  to  act,  cause 
wrinkled  seeds. 

I.  Law  of  Dominance. — What  Mendel  actually  found 

^  See  p.  423,  paragraph  376. 


554 


STRUCTURE    AND    LIFE    HISTORIES 


by  his  experiments  was  that,  in  such  a  cross,  all  the  seeds 
of  the  hybrid  plants  are  smooth.  The  inheritance  was 
"smooth"  and  "wrinkled,"  but  the  expression  was  of 
only   one   type — smooth.     A    character   thus   expressed, 


4  fc  4  4 


II II  nil 


ckI 


^ 


Fig.  407. — Mendelian  segregation  in  the  edible  pea  (Pisiim  sativum). 
Full  explanation  in  the  text.     (Cf.,  Fig.  408.'^ 

to  the  exclusion  of  another,  in  the  first  filial  (Fi)  genera- 
tion Mendel  called  dominant,  and  the  phenomenon  he 
called  dominance;  the  other  character  is  recessive.  From 
such  observations  Mendel  formulated  the  law  of  domi- 
nance, as  follows:     When  pairs  of  contrasting  characters 


EXPERIMENTAL    STUDY    OF   HEREDITY  555 

are  combined  in  a  cross,  one  character  behaves  as  a  dominant 
over  the  other,  which  is  recessive. 

By  similar  experiments  Mendel  found  that,  in  the  coty- 
ledons, yellow  is  dominant  over  green,  tallness  over  dwarf- 
ness,  axial  flowers  over  terminal,  and  so  on.  Such  pairs 
of  contrasting  characters  are  called  allelomorphs. 

2.  Law  0]  Segregation. — But  what  will  happen  if  the 
first  filial  (Fi)  generation  is  inbred  or  self-poUinated.  Its 
inheritance  included  factors  that  make  for  both  "smooth" 
and  "wrinkled,"  but  the  expression  was  all  of  one  kind 
only.  The  experiment  was  made,  and  Mendel  found  that 
the  second  fihal  (F2)  generation  included  plants,  part  of 
which  possessed  only  smooth  seeds,  while  the  others  had  only 
wrinkled  seeds  (Fig.  407).  "  Transitional  forms  were  not 
observed  in  any  experiment."  This  illustrates  in  a  strik- 
ing way  the  difference  between  inheritance  and  expression. 

475.  Ratio  of  Segregation. — But  now  we  come  to  that 
feature  of  Mendel's  experiments  which,  perhaps  more  than 
anything  else,  made  them  superior  to  all  others  that  had 
preceded.  He  carefully  counted  the  number  of  plants 
bearing  each  kind  of  seed,  and  found  that  the  number 
of  smooth-seeded  plants  was  to  those  with  wrinkled 
seeds  as  3  :i. 

476.  Theory  of  Ptirity  of  Gametes. — When  the  wrinkled 
seeds  (one-fourth  of  the  total  crop)  were  sown  they  all 
bred  true  to  wrinkledness — their  descendants  of  the  F3 
generation  bearing  only  wrinkled  seeds.  The  expression 
was  alike  in  every  case.  The  gametes  that  united  to 
produce  these  plants  were  therefore  considered  pure  for 
^^ wrinkledness;^^  that  is,  it  was  inferred  that  they  did 
not  carry  any  inheritance  tending  to  produce  smoothness 
of  seed. 


556 


STRUCTURE    AND    LIFE    HISTORIES 


Fig.  408. — Mendelian  segregation  in  maize,  a,  the  starchy  parent;  6, 
the  sweet  parent;  C,  the  first  hybrid  {Fi)  generation,  produced  by  crossing 
a  and  h,  showing  the  dominance  of  starchiness;  d,  the  second  hybrid  (F2) 
generation,  showing  the  segregation  of  starchiness  and  sweetness  with  the 
ratio  of  three  starchy  to  one  sweet  (wrinkled)  grain.  Lower  row,  daughters 
of  d;  e,  /,  and  g  resulted  from  planting  starchy  grains.  One  ear  in  three  is 
pure  starchy,  the  other  two  mixed;  h,  result  of  planting  sweet  (wrinkled) 
seeds.  They  are  pure  recessives,  and  the  ear  is  pure  sweet.  (After  East.) 
(Cf.  Fig.  407.) 


EXPERIMENTAL    STUDY    OF   HEREDITY  557 

477.  Not  All  Dominants  Alike.— But  when  the  seeds  of 
the  F2  plants,  having  only  smooth  seeds,  were  sown  it 
was  found  that  the  dominants  were  not  alike,  except  in 
external  appearance.  The  seeds,  though  all  appeared 
smooth,  carried  different  inheritances.  One-third  of 
them  {i.e.,  one-fourth  of  all  the  seed  produced  by  the  F2 
generation)  bred  true  to  smoothness,  being  therefore  pure, 
or  homozygous,  for  smoothness;  the  other  two-thirds  of 
the  dominants  {i.e.,  one-half  of  all  the  seed  produced) 
again  segregated  in  the  ratio  of  3 :  i — one-fourth  wrinkled 
and  three-fourths  smooth,  showing  that  they  were  hetero- 
zygous; that  is,  that  they  still  carried  inheritance  from 
both  the  wrinkled  and  smooth-seeded  grandparents. 

If  we  designate  the  first  parental  generation  by  P,  the 
dominant  character  (whatever  it  may  be)  by  D,  and  the 
recessive  character  by  R,  then  the  facts  above  described 
may  be  diagrammed  as  follows: 

D9  X  Rd'  P      (ist  Parental  generation) 

D  (R)  Fi     (ist  Hybrid  generation) 

4 


1  T 

3D  iR     F2     (2d  Hybrid  generation) 


iD  +  2D(R) 


D  3D        iR    R      Fs     (3d  Hybrid  generation) 

478.  Significance  of  the  Mendelian  Ratio. — The  ratio 
3  :  I  or,  as  it  appears  on  analysis,  i  :  2  :  i,  is  the  ratio  that 
one  might  expect,  or  that  might  be  predicted,  on  the  basis  of 
chance.  Students  of  algebra  will  recognize  in  it  the  essence 
of  the  familiar  square  of  a  +  ^,  namely,  a^  +  2ah  +  6^, 


558  STRUCTURE   AND    LIFE   HISTORIES 

where  a  and  h  each  equal  i.  In  the  plants  the  multi- 
plication of  inheritances  (produced  in  fertilization)  was 
as  follows: 

eggs  {s  +  7C')  X  sperms  {s  +  ^^0  =  ^^  +  2sw  -\-  ww 

where  w  =  wrinkling  and  s  =  absence  of  wrinkling,  i.e., 
smoothness. 

479.  Theory  of  Purity  of  Gametes. — The  above  ratio 
is  what  we  would  expect  if  half  of  the  egg-cells  and  half 
of  the  sperm-cells  in  a  heterozygous  plant  (one  of  the  Fi 
generation),  carried  only  character-units  or  determiners^ 
that  make  for  smoothness;  the  other  half  only  those 
factors  that  make  for  wrinkling,  giving  5  and  w  egg-cells, 
and  s  and  w  sperm-cells  in  equal  numbers.  Therefore,  in 
pollination  the  chances  would  be  equal  that  an  s-egg  would 
be  fertilized  with  either  an  5-sperm  or  a  w-sperm,  giving 
{s -\- w)  X  {s  -\-  w)  =  ss  -{-  2SW  -\-  WW.  Since  s  is  dominant 
over  w  the  product  should  be  written: 

ss  -j-  siii")  +  s{w)  -\-  ivw 

giving  in  external  appearances  35  +  iw.  Since  the  re- 
sult actually  observed  is  what  it  would  be  ij  the  gametes 
were  thus  ''pure"  for  smoothness  and  wrinkling,  Mendel 
concluded  that  they  really  are,  and  moreover  that  each 
character  behaves  as  a  unit,  appearing  and  disappearing 
in  its  entirety. 

480.  Character -units  versus  Unit-characters. — As  just 
stated,  Mendel  held  that  the  various  visible  characters  of 
his   plants   (dwarfness,  for  example)   behaved  as  units, 

^  The  substance  or  condition  (protoplasmic  constitution),  whatever  it  is, 
in  the  germ-cells  that  corresponds  to  any  given  character  of  the  plant  is 
variously  referred  to  by  the  terms  factor,  determiner ,  gene  (=  producer), 
character-unit,  and  others.    These  terms  are  essentially  synonyms. 


EXPERIMENTAL   STUDY    OF    HEREDITY  559 

either  appearing  in  their  fullness,  or  not  appearing  at  all. 
From  more  careful  observations  we  know  that  such  is 
not  the  case.  A  blossom  may,  for  example,  be  more  or 
less  pink,  an  odor  more  or  less  strong,  dwarfs  are  not 
all  the  same  height,  but  fluctuate  around  a  mean.  We 
conclude  therefore  that  characters  do  not  behave  as 
units,  and  that  the  conception  of  ''unit-characters"  is 
erroneous.  The  evidence  does,  however,  seem  to  justify 
the  conclusion  that  the  factor  or  factors,  whatever  they 
may  be,^  that  are  causally  related  to  the  given  character 
do  behave  as  units.  We  may  therefore  designate  them 
as  character -units.  They  are  commonly  known  as  genes. 
Quite  probably,  in  many  if  not  all  cases,  more  than  one 
gene  or  character-unit  is  involved  in  the  production  of 
any  given  character. 

481.  Applications  of  MendePs  Law. — Over  loo  pairs 
of  structural  and  color  characters  have  been  found,  in 
plant  breeding,  to  behave  more  or  less  closely  in  accord- 
ance with  the  Mendelian  conception.  In  peas  alone  over 
20  pairs  of  characters  are  expressed  in  successive  genera- 
tions, in  accordance  with  this  law.  Among  the  more 
striking  results  which  are  explainable  upon  Mendelian 
theory  are  the  following: 

1.  Mottled  beans  have  been  produced  in  the  Fi  genera- 
tion by  crossing  two  varieties,  neither  of  which  had  mottled 
seeds.     Vaiious  types  appeared  in  the  F2  generation. 

2.  Jet  black  beans  have  appeared  in  the  Fi  generation 
from  a  cross  between  two  varieties,  one  of  which  had  pure 
white  seeds,  the  other  light  yellow.  Various  shades  and 
colors  appeared  in  the  F2  generation. 

3.  In  one  case  three  distinct  varieties  of  beans,  breeding 

*  Substance  or  condition,  we  know  not  what,  within  the  germ-cells. 


560  STRUCTURE    AND    LIFE    HISTORIES 

true  to  white  seeds  (when  selfed^),  were  crossed  with  the 
same  variety  of  red  bean.  In  the  Fi  generation  each  cross 
gave  a  different  color — one  blue,  another  black,  and  the 
third  brown.  A  varied  assortment  of  colors  appeared  in 
each  case  in  the  F2  generations. 

4.  Two  varieties  of  sweet  peas,  each  breeding  true  to 
white  flowers,  when  crossed  gave,  in  the  Fi  generation, 
nothing  but  purple-flowered  ofTspring,  resembling  the  wild 
sweet  pea.  A  medley  of  white,  pink,  purple,  and  red- 
flowered  plants  appeared  in  the  F2  generation.  Numer- 
ous other  cases  might  be  cited,  all  of  which  would  have 
been  unsolvable  riddles  except  in  the  light  of  Mendelism. 

482.  Inheritance  and  Environment.— Emphasis  should 
be  laid  on  the  fact  that  the  behavior  of  any  plant,  and 
the  characters  it  manifests,  are  the  result  of  the  combined 
influence  of  inheritance  and  environment.  A  bean  seed- 
ling is  green,  not  merely  because  it  has  inherited  chloro- 
plastids,  but  because  it  develops  in  sunlight;  without 
sunlight  the  green  color  could  not  come  into  expression. 
If  we  vary  any  factor  of  environment  (temperature,  mois- 
ture, illumination,  food)  the  expression  of  the  inheritance 
may  be  altered,  just  as  truly  as  though  the  inheritance 
were  changed.  The  characteristics  expressed  by  any  plant 
{or  animal)  are  the  result  of  the  combined  action  of  inherit- 
ance and  environment.  It  is  of  fundamental  importance 
to  a  man,  not  only  to  be  ''well-born"  {eugenics),  but  also 
to  be  ''well-placed"  {euthenics). 

483.  Johannsen's  Conception  of  Heredity. — The  con- 
ception that  inheritance,  as  previously  noted,  is  not  the 
transmission  of  external  characters  from  parent  to  off- 
spring, but  the  reappearance,  in  successive  generations, 

^  The  pollination  of  a  flower  with  its  own  pollen,  or  with  pollen  from 
another  flower  of  the  same  plant,  is  called  selfing. 


EXPERIMENTAL    STUDY   OF   HEREDITY  561 

of  the  same  organization  of  the  protoplasm  with  reference 
to  its  character-units,  was  first  developed  by  Johannsen, 
of  Copenhagen,  Denmark,  who  proposed  the  term 
''genes/'  ''The  sum  total  of  all  the  'genes' in  a  gamete 
or  zygote,"  is  a  genotype.  Inheritance  is  the  recurrence, 
in  successive  generations,  of  the  same  genotypical  constitu- 
tion of  the  protoplasm.  Johannsen  does  not  attempt  to 
explain  the  nature  of  the  genes,  "but  that  the  notion 
'gene'  covers  a  reality  is  evident  from  Mendelism." 

This  conception  of  heredity  is  diametrically  opposed 
to  the  older  and  popular  conception,  but  is  much  more 
closely  in  accord  with  the  facts  revealed  by  recent  studies 
of  plant  and  animal  breeding.^ 

484.  Value  of  MendePs  Discoveries. — The  discoveries 
that,  in  inheritance,  certain  characters  are  dominant 
over  certain  others;  that  a  given  inheritance  {e.g.,  condi- 
tions associated  with  seed-color,  odor,  eye-color,  stature, 
musical  ability,  insanity,  tendency  to  some  disease)  may 
be  carried  and  transmitted  to  offspring  by  an  adult 
who  gives  no  outward  signs  of  carrying  the  inheritance; 
that,  under  certain  conditions  of  breeding,  some  characters 
(the  recessive  ones),  whether  good  or  bad,  may  become 
permanently  lost;  that  dominant  characteristics  are 
certain  to  appear  in  some  of  the  offspring — all  of  these 
truths,  learned  by  the  study  of  a  common  garden  vegetable, 
will  be  recognized  at  once  as  of  enormous  importance  to 
the  breeders  of  plants  and  animals,  and  above  all  to  man- 
kind, in  connection  with  our  own  heredity.  They  point 
the  way  to  the  explanation  of  such  enigmas  as  the  pro- 
verbial bad  sons  of  pious  preachers,  spendthrift  children 

^  A  discussion  of    Johannsen's  very  fruitful    method  of  "pure  line" 
breeding  belongs  to  more  advanced  studies. 
36 


562  STRUCTURE    AND    LIFE    HISTORIES 

of  thrifty  parents,  talented  offspring  of  mediocre  parents, 
blue-eyed  cliildren  of  brown-eyed  parents,^  and  so  on. 

485.  Increased  Vigor  from  Crossing. — Experiments 
with  pedigreed  cultures  have  disclosed  a  principle  of  the 
utmost  practical  importance  for  the  plant  breeder.  A 
careful  analysis  of  a  field  of  Indian  corn  {Zea  Mays)  has 
disclosed  the  fact  that  any  given  variety  is  very  complex, 
being  heterozygous  for  many  characters;  in  other  words 
any  horticultural  variety  is  a  composite  of  numerous 
elementary  species,  and  is  therefore  heterozygous  for 
most  of  its  characters.  When  pollination  is  allowed  to 
take  place  in  the  corn  field  without  interference  by  man, 
both  crossing  and  selfing  occur.  As  a  result  the  yield, 
in  bushels  per  acre,  remains  about  stationary,  or  gradu- 
ally becomes  less  and  the  variety  changes  and  deterior- 
ates by  the  segregation  and  recombination  of  the  numerous 
elementary  species  that  compose  it. 

By  artificial  self-pollination  for  several  generations  {e.g., 
five  or  more)  less  complex  strains  result,  which  are  homo- 

^  If  both  parents  have  blue  eyes  the  children  can  never  have  brown  eyes; 
if  one  parent  has  brown  eyes  and  one  blue,  the  children  may  be  both  blue- 
and  brown-eyed,  or  all  brown-eyed,  for  brown  eye-color  in  man  is  dominant 
over  blue  color.  "When  both  parents  have  brown  eyes,  part  of  the  children 
may  have  blue  eyes  and  part  of  them  brown,  or  they  may  all  be  brown- 
eyed.  As  used  here,  the  term  "brown  eyes"  means  all  eyes  having  brown 
pigment,  whether  in  small  spots  (gray  eyes),  or  traces  (hazel  eyes),  or 
generally  distributed  (brown,  or  sometimes  black,  eyes).  The  term  "blue 
eyes"  designates  only  those  cases  in  which  brown  pigment  is  entirely  lacking. 

Fig.  409. — Zea  Mays.  In  the  experiment,  the  results  of  which  are  here 
illustrated,  nine  strains  of  Indian  corn  were  selected  according  to  the 
number  of  rows  of  kernels  on  the  cob,  varying  from  8  to  24  rows.  These 
were  pollinated  by  hand  each  year,  with  mixed  pollen,  in  such  manner  that 
self-pollination  was  entirely  prevented.  An  average  ear  of  each  strain  is 
shown  in  the  first  row  above.  In  the  second  row  is  shown  an  average 
ear  of  each  strain  after  self-fertilization  for  five  generations.  Note  the 
resulting  decrease  in  the  number  of  rows,  lack  of  filling  out  of  the  ears, 
and  other  marks  of  inferiority.  The  last  row  shows  the  remarkable  and 
immediate  increase  of  vigor  resulting  in  the  Fi  generation  of  hybrids  be- 
tween various  pairs  of  the  selfed  strains.     (Photo  supplied  by  G.  H.  Shull.) 


EXPERIMENTAL    STUDY    OF   HEREDITY 


563 


564  STRUCTURE    AND    LIFE    HISTORIES 

zygous  for  one  or  more  characters,  and  the  yield  per  acre 
may  thus  become  greatly  reduced.^  If  now,  two  of  these 
simplified  strains,  homozygous  for  many  characters,  and 
having  a  low  yield  per  acre,  are  crossed,  there  results  an 
Fi  hybrid  progeny  that  is  heterozygous  for  all  of  these 
characters.  This  heterozygosity  is  correlated  with  a 
greatly  increased  vigor;  the  plants  are  much  larger,  and 
the  yield  per  acre  is  enormously  increased  (Fig.  409). 
Thus  in  one  experiment  of  this  kind  the  average  yield  of 
the  heterozygous  horticultural  variety  was  61.25  bushels 
per  acre.  After  self-fertilization  for  several  generations 
the  yield  became  reduced  to  29.04  bushels  per  acre;  but 
in  the  Fi  generation  of  a  cross  between  two  of  these  self- 
fertiUzed  strains  the  yield  per  acre  rose  at  once  to  68.07 
bushels.  In  the  F2  generation  the  yield  again  fell  to  44.62 
bushels.  From  this,  and  numerous  other  experiments,  it 
is  found  that  the  biggest  corn  crop  is  to  be  obtained  by 
finding  the  strains  that  will  produce  the  largest  yield 
when  crossed,  and  thus  using  for  seed  the  grains  of  the 
first-generation  hybrids  each  year. 

486.  Unsolved  Problems. — ^Like  all  truly  great  con- 
tributions to  science,  MendeFs  discoveries  have  raised 
more  questions  than  they  have  answered.  Therein  lies, 
in  part,  their  great  value.  So,  also,  the  most  important 
effect  of  Darwin's  work  was  that  it  set  men  to  asking 
questions.  The  history  of  botany,  as  of  all  natural  science 
since  1859,  is  chiefly  the  attempts  of  men  to  answer  the 
questions  raised  by  Darwin,  or  stimulated  in  their  own 
minds   by   his   books.     So   with   Mendel   and  de  Vries; 

^  If  a  high-yielding  strain  was  separated  out  by  selection,  the  yield 
would  of  course  be  increased  above  the  average  of  the  mixed  field. 


EXPERIMENTAL    STUDY    OF   HEREDITY  565 

biological  science,  since  1900,  has  been  largely  occupied 
in  trying  to  answer  the  questions  raised  by  these  men. 

What  are  these  questions?  There  is  not  space  here 
even  to  ask  them  all,  much  less  to  endeavor  to  answer 
them  even  briefly;  but  they  include  the  following  large 
problems : 

1.  What  is  the  mechanism  of  inheritance?  In  other 
words,  by  what  arrangement  and  interaction  of  atoms 
and  molecules  is  it  made  possible  that  the  peculiar  tone 
of  one's  voice,  the  color  of  a  rose,  the  odor  of  a  carnation, 
the  evenness  (or  otherwise)  of  one's  disposition,  may  be 
transmitted  from  one  generation  to  another?  How  may 
it  be  transmitted  through  one  generation,  without  causing 
any  external  expression,  and  reappear  in  the  second  genera- 
tion removed?  Is  the  cytoplasm  the  carrier,  or  the 
chromatin,  or  both  combined,  or  neither?  Is  the  transfer 
accomplished  by  little  particles  (pangens),  as  de  Vries 
contends,  or  by  chondriosomes,  or  otherwise?  We  do 
not  know. 

2.  How  may  dominance  he  explained?  Why  is  tallness 
dominant  over  dwarfness,  brown  eye-color  over  blue, 
any  one  character  over  any  other?  We  have  not  the 
faintest  idea. 

2,.  Are  acquired  characters  inherited?  In  other  words, 
do  characteristics  acquired  after  birth  by  the  body  or 
mind  of  the  parent,  either  by  its  own  activity  or  as  a  re- 
sult of  the  immediate  effects  of  environment,  influence 
the  germ-cells  so  as  to  alter  the  inheritance  which  they 
transmit?  Some  say  yes,  others  say  no;  others  say, 
only  in  part.  There  seems  to  be  evidence  both  ways. 
We  can  arrive  at  the  correct  answer  only  by  careful 
experimentation,  that  is,  by  asking  questions  of  nature. 


566  STRUCTURE    AND    LIFE    HISTORIES 

4.  Can  the  inheritance  of  a  strain  he  artificially  altered? 
This  is  a  question  of  the  very  first  importance.  If  the 
inheritance  could  be  so  altered  the  marvels  that  breeders 
might  perform  would  be  greatly  increased.  A  blue  rose 
(the  despair  of  all  plant  breeders)  might  possibly  be  pro- 
duced by  sufficiently  careful  and  extended  experiment- 
ing; disease  and  undesirable  traits  of  character  might  be 
eliminated  from  certain  tainted  family  strains  by  artificial 
treatment.  On  the  other  hand,  by  an  unfortunate  com- 
bination of  circumstances,  most  undesirable  and  re- 
grettable results  might  be  experimentally  produced.  The 
experiment  has  been  made  of  exposing  the  ovaries  of 
flowers  to  the  rays  of  radium,  and  of  injecting  them  with 
various  chemical  substances,  with  an  idea  of  altering  the 
physical  or  chemical  nature  of  the  egg-cells,  and  thus 
altering  the  inheritance.  The  results  of  such  experiments, 
so  far  tried,  need  to  be  further  confirmed  before  we  can 
say  with  certainty  that  the  result  sought  has  been 
accomplished. 

487.  Eugenics. — Students  of  biology  have  been  quick 
to  recognize  the  fact  that,  if  we  correctly  understand  the 
laws  of  heredity,  we  are  in  a  position  to  apply  them,  not 
only  to  plants  and  the  lower  animals,  but  to  mankind. 
The  application  of  the  laws  of  heredity  in  a  way  to  pro- 
duce a  healthier  and  more  efficient  race  of  men  constitutes 
the  practice  of  eugenics}  The  underlying  principles  of 
eugenics  are  of  course,  very  largely  those  of  heredity. 
Eugenics  is  the  applied  science  based  upon  the  pure 
science  of  heredity.  The  main  problem  of  eugenics  is 
how  to  eliminate  human  beings  with  a  tendency  to  any 
physical  or  mental  weakness,  making  for  poverty,  misery, 

*  Eugenics  is  from  two  Greek  words  meaning  well  born. 


EXPERIMENTAL    STUDY   OF   HEREDITY  567 

ignorance,  and  crime;  and  how  to  increase  the  number 
of  individuals  physically,  mentally,  and  morally  more 
robust  and  sound;  and  withal  how,  if  possible,  to  raise 
the  standard  of  all  desirable  human  traits.  A  careful 
study  of  heredity  and  eugenics  will  make  possible  a  much 
more  intelligent  and  efficient  program  for  charity  work 
and  social  betterment. 

488.  Investigations  Since  Mendel. — It  must  be  re- 
membered that  Mendel's  most  valued  contribution  was 
not  the  observations  which  he  made  and  recorded  con- 
cerning the  garden  pea,  nor  the  hypotheses  which  he  ad- 
vanced on  the  basis  of  those  observations,  but  this  method 
of  procedure,  whereby  he  elevated  the  study  of  heredity 
to  the  rank  of  an  exact  science.  As  in  the  case  of  all 
hypotheses,  the  task  for  science  is  to  subject  them  to  the 
most  searching  tests,  to  see  if  they  invariably  agree  with 
facts,  and  may  be  accepted  as  in  all  probability  embody- 
ing the  actual  truth — may  be  elevated  to  the  rank  of 
theories.  The  testing  of  Mendelism  has  been  occupying 
all  the  best  talents  of  many  investigators  since  the  re- 
discovery of  Mendel's  publication,  about  1900.  Many 
biologists  are  still  skeptical,  others  reject  the  hypotheses, 
and  still  others  believe  they  contain  the  germ  of  truth, 
but  must  be  more  or  less  modified.  Whether  they  prove 
to  he  erroneous  or  true  is  not  so  important,  hut  it  is  impor- 
tant for  us  to  know  which  is  the  case.  True  or  not, 
they,  like  nearly  all  working  hypotheses  (natural  selec- 
tion, mutation,  nebular  hypothesis,  atomic  hypothesis 
in  chemistry,  etc.)  are  rendering,  or  have  rendered,  a 
priceless  service  to  science  by  pointing  the  way  to  further 
study,  which  enriches  our  knowledge  of  the  living  world, 
including  ourselves,   and   therefore  increases   the  intelli- 


568  STRUCTURE    AND    LIFE   HISTORIES 

gence  with  which  we  may  order  our  own  conduct  and  lives. 
If  the  study  of  plants  had  rendered  no  other  service  to 
mankind  than  this  contribution  of  an  effective  method  of 
ascertaining  the  laws  of  heredity,  it  would  have  amply 
justified  all  the  arduous  labor  that  men  have  devoted  to 
it  for  2,000  years. ^ 

^  Only  one  of  the  simplest  cases  worked  out  by  Mendel  is  summarized 
in  this  chapter.  A  more  thorough  study  of  his  experimental  results  and 
theories  must  be  reserved  for  a  more  advanced  course. 


CHAPTER  XXXVI 
THE  EVOLUTION  OF  PLANTS 

489.  The  Problem  Stated. — If  we  knew  the  entire 
history  of  development  of  the  plant  world,  we  could  ar- 
range all  plants  now  living,  and  that  have  lived,  so  as  to 
show  their  genetic  relation  to  each  other.  The  prob- 
lem is  illustrated  on  a  small  scale  by  various  related  culti- 
vated plants,  all  known  to  be  derived  from  a  common 
wild  ancestor.  Cabbage  and  its  relatives  are  a  case  in 
point.  The  botanical  relatives  of  the  cabbage  include 
such  forms  as  kohlrabi,  brussels-sprouts,  collards,  kale, 
broccoli,  and  cauliflower  (Fig.  397).  All  of  these  garden 
vegetables  are  believed  to  have  been  derived  from  the 
common  wild  cliff-cabbage  {Brassica  oleraced)  of  Europe 
and  Asia,  by  selecting  mutations  in  various  directions, 
e.g.,  excessive  development  of  the  stem  in  kohlrabi,  of 
the  terminal  bud  in  cabbages,  of  the  lateral  buds  in  brus- 
sel's  sprouts,  of  the  flower  buds  in  cauliflower.  Or,  to 
refer  to  de  Vries's  studies  in  experimental  evolution,  where 
the  course  of  descent  was  actually  observed,  we  may 
arrange  the  forms  of  Lamarck's  evening-primrose  so  as 
to  show  their  known  derivation. 

The  general  problem,  therefore,  is  to  establish  the 
genetic  relationship  of  all  known  plants,  living  and  fossil. 
Since  the  Angiosperms  stand  at  the  top  of  the  series,  the 
problem  resolves  itself  largely  into  ascertaining  the 
phytogeny,  or  line  of  ancestry,  of  that  group. 

569 


570  STRUCTURE    AND    LIFE    HISTORIES 

490.  Methods  of  Study. — In  the  solution  of  this  prob- 
lem two  methods  of  attack  may  be  employed:  (i)  That 
of  observation  and  comparison  of  structure,  followed  by 
classification,  and  inference;  (2)  that  of  experiment.  The 
use  of  experiment  is  indicated  in  Chapters  XXXIII  and 
XXXV.  By  this  means  we  may  learn  something  of  the 
relationship  of  different  groups  having  living  representa- 
tives; but  it  chiefly  serves  to  throw  light  on  the  method  of 
evolution.  The  course  of  evolution  is  best  ascertained  by 
the  observation  and  comparison  of  plant  structures. 

491.  Sources  of  Evidence. — There  are  four  main  sources 
of  evidence  as  to  the  course  of  evolution: 

1.  Comparative  anatomy  of  living  forms. 

2.  Comparative  life  histories  of  living  forms. 

3.  Structure  of  fossil  forms. 

4.  Geological  succession  of  fossil  forms. 

Studies  along  these  four  different  lines  have  resulted 
in  some  conflict  of  evidence,  but  on  the  whole  the  evidence 
from  the  various  sources  all  points  to  the  same  broad 
conclusions.  Conflict  or  contradication  is  in  most  cases 
the  result  of  insufficient  evidence  from  one  or  more  sources. 

492.  Evidence  from  Comparative  Anatomy. — Compara- 
tive study  of  structure  has  led  to  the  conclusion  that, 
in  its  broadest  aspects,  the  course  of  plant  evolution  has 
been  from  the  simple  to  the  complex;  that  such  simple 
organisms  as  Fleurococcus ,  and  other  green  algae,  preceded 
more  complex  forms  like  the  liverworts ;  that  Bryophytes 
appeared  before  ferns,  and  they  in  turn  before  the  modern 
Gymnosperms  and  Angiosperms. 

A  difficulty  of  accepting  this  conclusion  as  final  is  the 
possibility  that,  at  certain  points,  the  course  of  evolution 
may  have  been  retrograde.     For  example  it  is  generally 


THE    EVOLUTION    OF   PLANTS  571 

accepted  that  the  filamentous,  alga-like  fungi  were  de- 
rived from  green  algae  by  retrograde  evolution  (degenera- 
tion).    Were  the  plants  with  one  seed-leaf  (monocoty- 
ledons) derived  from  those  with  two  (dicotyledons)  by 
retrograde  evolution,  or  were  the  dicotyledons  derived 
from  the  monocotyledons  by  progressive  evolution?     Evi- 
dence, recently  ascertained  by  studies  of  structure  and 
development,   points  to   the  conclusion,   that,   although 
monocotyledony  seems  the  simpler,  more  primitive  con- 
dition, it  is  really  a  later  phenomenon,  the  monocotyledons 
being  derived  from  the  dictoyledons  by  simplification. ^ 
Again,  a  careful  student  of  fossil  plants  has  recently  been 
led  to  state  that,  ''it  is  beginning  to  appear  more  probable 
that  the  Higher  Cryptogams  (ferns  and  fern  allies)  are  a 
more  ancient  and  primitive  group  than  the  Bryophytes, 
which  would  seem  to  owe  'their  origin  to  reduction  from 
some  higher  type."^     In  view  of  this  diversity  of  opinion, 
we  learn  at  once  that  great  caution  must  be  used  in  in- 
terpreting  the   evidence— that  we  must  not  ''jump  at 
conclusions." 

493.  Results  of  the  Method  of  Comparative  Anatomy. 
— By  their  study  of  comparative  anatomy  and  morphol- 
ogy, botanists  have  been  led  to  propose  the  following 
arrangement  of  plant  groups  as  representing  the  general 
course  of  their  evolution  (Table  V) : 

From  what  has  already  been  said,  however,  it  should 
be  understood  that  such  a  table  represents,  not  the  line 
of  evolutionary  advance,  but  the  paths  travelled  by  plants 
in  the  course  of  their  development.  For  example,  it  implies 
that   dicotyledons   were   derived   from   monocotyledons, 

1  See  paragraph  522,  Chapter  XXXVIII. 

2  Scott,  D.  H.     "The  Evolution  of  Plants,"  p.  18. 


572  STRUCTURE    AND    LIFE    HISTORIES 

Table   V. — Sequence  of   Plant   Groups,  Based   on  the 
Morphology  of  Living  Forms 

Thallophytes  /  AlgjE — having  chlorophyll, 

(no  archegonia)  \  Fungi — no  chlorophyll. 

j  Bryophytes — no  vascular  system. 
Archegoniates  J  Pteridophytes    ] 

farchegonia,  but  no  seeds)    ]  Calamophytes   \  vascular  system. 
Lepidophytes 


I 


Spermatophytes 
(seeds) 


Gymnosperms — no  closed  ovary. 
Angiosperms — closed  ovary  (pistil). 

Monocotyledons — one  seed-leaf. 

Dicotyledons — two  seed-leaves. 


pteridophytes  from  bryophytes — hypotheses  which,  from 
other  trustworthy  evidence,  as  stated  above,  now  seem 
untenable. 

Again,  the  table  suggests  that  Angiosperms  were  de- 
rived from  Gymnosperms,  and  therefore  appeared  late 
in  the  history  of  plant  life;  but  the  study  of  fossil  plants 
shows  that  they  appeared  in  the  geological  past,  and  were 
dominant  in  the  Tertiary  period,  as  now.  We  are  led, 
therefore,  to  proceed  with  caution  in  drawing  inferences 
based  only  upon  a  comparative  study  of  the  structure  of 
forms  now  living. 

494.  Evidence  from  Life  Histories. — In  the  study  of 
the  life  history  (ontogeny)  of  any  higher  sporophyte, 
we  find  that  vegetative  (sterile)  tissues  develop  first. 
On  the  basis  of  this  fact  it  has  been  inferred  that  all  repro- 
ductive organs  (stamens,  carpels,  sporophylls)  arose  by 
a  modification  of  vegetative  organs.  Other  facts,  how- 
ever, lead  to  the  directly  opposite  conclusion. 

495.  Evidence  from  Comparative  Ontogeny. — In 
Chapters   XVI   and   XXIII   attention   is   called   to   the 


THE    EVOLUTION    OF    PLANTS  573 

fact  that  the  most  primitive  sporophytes  of  the  lower 
liverworts  consist  almost  entirely  of  ''fertile"  {i.e.,  re- 
productive) cells,  and  that  the  relative  amount  of  vege- 
tative or  sterile  tissue  gradually  increases  (as  we  pass  to 
more  highly  organized  forms)  by  a  progressive  sterilization  of 
fertile  tissue.  This  progressive  sterihzation  accompanied 
a  change  in  the  habitat  of  plants  from  water  to  dry  land. 

496.  Consequences  of  an  Amphibious  Habit  of  Life . — 
The  Hfe  history  of  the  fern  affords  a  concrete  illustration. 
The  gametophyte  is  semi-aquatic  in  habit,  and  the  method 
of  fertiHzation  is  purely  aquatic,  the  sperm  being  unable 
to  reach  the  egg  except  by  swimming  through  free  water. 
But,  when  the  fertihzed  egg  began  to  develop  as  a  land 
plant,  the  chances  of  fertiHzation  by  a  sperm  swimming 
in  free  water  became  increasingly  remote.  The  perpetua- 
tion of  the  species,  and  the  multiplication  of  individuals 
could  be  insured  only  by  the  formation  of  a  large  number  of 
reproductive  bodies  (spores),  capable  of  distribution  by 
wind  in  dry  conditions,  and  each  able  to  reproduce  its 
kind  independently,  without  fusion  with  another  repro- 
ductive body.  The  larger  the  number  of  such  spores,  the 
greater  the  chances  of  perpetuation  of  the  given  species. 

497.  Consequence  of  Enormous  Spore -production. — 
But  the  formation  of  a  large  number  of  spores  requires  a 
vigorous  plant  body  to  supply  them  with  an  abundance 
of  water  and  nourishment,  and  to  lift  them  up  into  the 
air  where  they  would  stand  a  better  chance  of  distribution 
when  dry.  This  is  accomplished  by  the  sporophyte,^ 
producing  an  abundance  of  broad,  green  leaves  for  food- 

^  "The  fern,  as  we  normally  see  it,  is  an  organism  with,  so  to  speak, 
one  foot  in  the  water,  the  other  on  the  land."  Bower,  F.  O.,  "The  Origin 
of  a  Land  Flora,"  p.  82, 


574  STRUCTURE    AND    LIFE    HISTORIES 

manufacture,  and  of  roots  for  absorption  of  water  and 
minerals  in  large  quantities.  From  this  point  of  view, 
the  plant  body  of  the  sporophyte  is  regarded  as  produced 
by  the  progressive  sterilization  of  tissues  originally  re- 
productive. After  the  formation  of  a  vigorous  plant 
body  then  spores,  produced  in  special  regions  (sporangia) , 
could  be  nourished  in  enormous  numbers. 

498.  Origin  of  Vegetative  Organs. — On  the  basis  of 
the  theory  just  outlined,  we  are  to  regard  foliage-leaves 
and  branches,  either  as  new  formations ^  developed  (by 
"enation^^)  on  some  primitive  reproductive  axis  like  a 
strobilus  or  cone,  or  else  as  produced  by  the  sterilization 
of  parts  originally  fertile,  i.e.,  modifications  of  reproductive 
tissues.  The  sporophyte,  as  we  have  already  seen,^ 
has  become  increasingly  well  developed  and  increasingly 
independent,  while  the  gametophyte  has  become  increas- 
ingly simple  and  increasingly  dependent.  The  evolution  of 
plants  has  proceeded  by  the  progressive  development 
of  the  sporophyte,  and  the  gradual  but  steady  regres- 
sion of  the  gametophyte. 

499.  Steps  in  the  Evolution  of  the  Sporophyte. — The 
possible  steps  in  the  evolution  of  the  sporophyte  may  be 
tabulated  as  follows:^ 

1.  Sterilization  of  fertile  tissue. 

2.  Localization  of  spore-production  in  sporangia. 

3.  Origination  of  lateral  organs  (leaves),  and  of  roots. 

4.  Development  of  heterospory. 

5.  Introduction  of  fertilization  by  the  pollen-tube 
(siphonogamy) . 

6.  Assumption  of  the  seed-habit. 

1  Chapter  XXIII. 

2  Following  F.  O.  Bower. 


THE  EVOLUTION  OF  PLANTS  575 

600.  Another  Hypothesis  of  Alternation  of  Genera- 
tions.— Some  of  the  facts  of  paleobotany  support  the 
hypothesis  that  the  modern  sporophyte  has  not  been 
gradually  developed  from  a  simple  structure  like  the  moss 
sporogonium  (derived,  in  turn,  by  progressive  steriliza- 
tion), but  that  the  gametophytic  and  sporophytic  stages 
were  at  the  first  vegetatively  or  somatically  equivalent; 
except  for  chromosome  number  (as  is  the  case  now,  for 
example,  with  Dictoyota^  and  Polysiphonia);  and  that, 
in  the  course  of  evolution,  the  sexual  phase  became  more, 
and  the  asexual  phase  less,  important  in  certain  forms 
{e.g. J  mosses),  and  the  asexual  phase  more,  and  the  sexual 
phase  less,  important  in  other  forms  (e.g.,  ferns).  This 
is  the  hypothesis  of  homologous  alternation,  as  opposed  to 
that  of  antithetic  alternation.^  The  structural  differences 
in  the  two  generations  are,  on  the  basis  of  this  hypothesis, 
considered  as  due  almost,  if  not  entirely,  to  differences 
in  environment,  the  main  factor  being  the  gradual  transi- 
tion from  aquatic  to  dry-land  surroundings.  Where  the 
environment  is  uniform  and  the  same  for  both  genera- 
tions, as  for  Dictyota,  the  gametophyte  and  sporophyte  are 
identical  in  external  organs  and  general  appearance  (Fig. 
177).  In  any  event  the  hypothesis  postulates  a  homology 
between  the  various  organs  of  the  two  generations,  how- 
ever much  these  parts  may  differ  in  external  appearance 
as  a  result  of  individual  variation  and  environmental 
influence. 

501.  Lang's  Ontogenetic  Hypothesis. — Viewing  the 
matter  from  the  standpoint  of  individual  development 
(ontogeny),  Lang  has  developed  the  ontogenetic  hypothesis 

1  See  Chapter  XVII. 
8  See  Chapter  XIV. 


576  STRUCTURE    AND    LIFE    HISTORIES 

oj  alternation.     From  this  point  of  view  two  alternatives 
are  recognized: 

1.  Either  the  fertilized  egg  and  the  haploid  spore  are 
potentially  unlike,  and  will  therefore  produce  unlike  plant 
bodies,  even  under  essentially  similar  environment,  or 

2.  Fertilized  eggs  and  spores  are  potentially  alike,  but 
produce  unlike  plant  bodies  as  the  result  of  the  difference 
in  the  enviromnent  in  which  they  develop. 

The  ontogenetic  school  accepts  the  latter  alternative 
as  a  working  hypothesis,  and  regards  the  gametophytic 
and  sporophytic  generations  as  essentially  homologous. 
The  degree  of  homology  which  can  actually  be  traced  in 
the  vegetative  structure  of  the  two  generations  may  vary 
from  substantial  identity,  as  in  Dictyota,  to  such  wide 
divergence  that  the  tracing  of  homologies  is  quite  out 
of  the  question.  In  testing  this  hypothesis  a  crucial 
experiment  would  be  to  obtain  a  gametophyte  by  arti- 
ficially bringing  a  fertilized  egg  to  mature  development 
outside  of  the  archegonium  and  under  the  environment  in 
which  the  spores  normally  develop;  or  to  obtain  a  sporo- 
phyte  by  causing  a  spore  to  develop  within  the  tissue  of  a 
gametophyte,  as  the  fertilized  egg  normally  does. 

502.  Hypothetical  Ancestral  Tree. — From  a  compara- 
tive study  of  both  living  and  fossil  forms  some  botanists 
have  been  led  to  infer  the  common  derivation  of  Filicales, 
Equisetales,  and  Lycopodiales  from  the  Hepaticse,  and 
probably  through  some  form  belonging  to  the  Anthocero- 
tales  somewhat  as  shown  in  the  following  ancestral  ''tree" 
(Fig.  410).  It  should  be  clearly  understood  that  this 
tree  does  not  illustrate  known  facts,  but  only  the  hypoth- 
eses which  have  been  tentatively  proposed  by  careful 
students  on  the  basis  of  known  facts. 


THE    EVOLUTION   OF   PLANTS 


577 


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Fig.  410. — Hypothetical  genealogical  tree  to  illustrate  the  probable  affini- 
ties of  the  modern  plant  orders.  This  diagram  is  intended  to  indicate  that 
the  plant  orders  now  existing  are  the  tips,  only,  of  the  branches  of  a  genealog- 
ical tree,  whose  lower  limbs  and  roots  extend  into  preceding  geological 
periods.  Our  knowledge  is  not  sufficient  to  enable  us  to  connect  these 
branches  with  each  other,  nor  with  the  main  trunk.  The  diagram  teaches 
that  hypothetical  (indicated  by  the  dotted  line)  Anthocerotales  gave  rise 
to  a  now  fossil  Filicalean  stock,  from  which  have  been  derived  all  the 
modern  orders  above  the  mosses  and  liverworts. 


37 


CHAPTER  XXXVII 
PALEOBOTANY 

503.  The  Scope  of  Paleobotany. — The  study  of  fossil 
plants,  though  of  course  a  phase  of  botany,  constitutes 
a  science  by  itself,  not  only  covering  a  special  subject 
matter,  but  having  its  own  methods  (technique),  and  pos- 
sessing a  large  literature.  It  is  called  paleobotany.  One 
cannot  pursue  this  study  without  a  knowledge  of  the 
anatomy  and  morphology  of  living  forms.  This  is  neces- 
sary in  order  to  interpret  the  meaning  of  plant  fossils, 
which  often  occur  only  in  small  fragments  of  the  entire 
plant.  Moreover,  one  must  have  a  good  knowledge  of 
at  least  the  elements  of  geology,  since  fossils  are  found  in 
rocks.  One  must  not  only  know  the  geological  age  to 
which  the  fossil-bearing  rock  he  studies  belongs,  but  also 
something  of  the  geological  processes  by  which  fossils, 
and  even  the  rocks  themselves,  are  formed. 

504.  What  is  a  Fossil? — A  fossil  is  any  remains  of  a 
plant  or  animal  that  lived  in  a  geological  age  preceding 
the  present;  these  remains  are  preserved  in  rocks. ^  There 
are  two  methods  of  preservation,  namely,  incrustation  and 
petrifaction.  Incrustations  are  merely  impressions  or 
casts  resulting  from  the  encasement  of  the  organ  or 
organism  in  the  rock-forming  material.     The  tissue  itself 

^  By  an  extension  of  the  term  we  also  speak  of  fossil  footprints  of  ani- 
mals, fossil  ripple  marks,  et  cetera.  The  word  fossil  is  derived  from  the 
LzX\nfodere  (to  dig),  and  originally  signified  anything  dug  up. 

578 


PALEOBOTANY  579 

either  decayed  or  became  carbonized,  leaving  only  the 
impression  of  its  surface  features.  The  well-known 
''fossil  fern-leaves,"  found  in  coal  mines  are  of  this  nature. 
The  tissues  of  the  plant  were  transformed  into  coal, 
leaving  the  impression  or  cast  on  the  adjacent  shale.  The 
first  stage  in  this  process  may  often  be  observed  in  the 
autumn,  when  impressions  of  recently  fallen  leaves  are 
made  on  the  surface  of  wet  mud.  Obviously  from  such 
fossils  we  can  learn  nothing  of  internal  structure. 

Petrifactions  are  formed  by  the  gradual  replacement 
of  the  organic  tissue  by  mineral  matter,  usually  carbonate 
of  lime  (CaCOs)  or  silicic  acid  (H4Si04).  In  this  process 
the  tissues  become  soaked  with  a  saturated  solution  of 
the  given  mineral,  which  is  gradually  deposited  from  solu- 
tion, and  takes  the  place  of  the  original  organic  matter. 
By  this  means  the  most  minute  details  of  microscopic 
structure  are  preserved,  even  in  some  cases  the  nuclei 
and  other  cell-contents. 

505.  Conditions  of  Fossil-formation. — In  order  to 
understand  how  fossils  come  to  be  formed,  we  must  pic- 
ture to  ourselves  certain  geological  processes  now  in 
operation — the  initial  stages  of  rock-formation.  Rocks 
are  of  two  kinds,  igneous  and  sedimentary.  Igneous  rocks 
result  from  the  cooling  of  molten  lava  poured  out  on 
the  surface  or  injected  into  crevices  by  volcanic  action. 
Such  rocks  never  contain  fossils,  as  the  intense  heat 
necessary  to  melt  the  rock  destroys  all  trace  of  organic 
matter. 

Sedimentary  rocks  are  formed  by  the  deposit  under 
water  of  the  sediment  formed  by  weathering  and  erosion, 
and  transported  by  streams.  This  deposit  may  occur 
along  the  flood-plains  or  at  the  mouths  of  streams  empty- 


58o 


STRUCTURE   AND   LIFE   mSTORIES 


PALEOBOTANY 


581 


ing  into  inland  lakes  or  into  the  ocean.     In  addition  to 
rock-sediment  eroded  from  the  surface  of  the  land,  streams 


^<i;&*Kl:<f£ 


Bf       Bs         Bm        5     M       O-W 


MF  Bf  Bs.Bm. 


A^l 


^y:^ 


iir^^fi 


'  />o^* 


Fig.  412. — Diagram  illustrating  the  gradual  filling  up  of  lakes  by  the 
encroachment  of  vegetation,  and  also  the  stages  in  the  origin  of  peat  and 
rnarl  deposits  in  lakes.  The  several  plant  associations  of  the  Bog  series, 
displacing  one  another,  belong  to  the  following  major  groups:  (i)  O.  W., 
open  water  succession;  (2)  M.,  marginal  succession;  (3)  S.,  shore  succes- 
sion; (4)  B.,  bog  succession,  comprising  the  bog-meadow  {Bm),  bog-shrub 
{Bs)  and  bog-forest  {Bf);  and  (5)  M.  F.,  mesophytic  forest  succession 
(Cf.  Fig.  411.) 

also  transport  quantities  of  plant  (and  animals)  frag- 
ments, leaves,  stems,  pieces  of  bark,  fruit,  flowers,  pollen 
and  spores,  roots,  and  even  entire  plants.     These  natur- 


582  STRUCTURE    AND    LIFE    HISTORIES 

ally  become  buried  in  the  mud  and  sediment  wherever 
deposition  takes  place,  and  when  the  deposit  becomes 
converted  into  rock  the  organic  remains  may  become  con- 
verted into  fossils  by  either  of  the  processes  described 
above.  Swampy  regions  are  especially  favorable  to  the 
preservation  of  plant  and  animal  remains  as  fossils,  as  is 
illustrated  in  Figs.  411  and  412. 

506.  Metamorphism. — After  sedimentary  rocks  are 
once  formed  they  are  subject  to  various  changes.  The 
amorphous  carbonate  of  lime,  of  limestone  rocks,  may  be 
transformed  into  crystals  of  calcite  until  marble  results; 
thin  flakes  of  mica  may  form  in  clay  rock  in  thin  sheets, 
transforming  the  rock  into  slate;  vegetable  deposits  in 
the  form  of  peat  may  become  transformed  into  anthracite 
coal  and  graphite;  molten  lava  poured  out  on  the  surface 
or  into  crevices  of  sedimentary  rocks  may  fuse  the  adja- 
cent material,  causing  contact  pietamorphism;  while  the 
heat  engendered  over  larger  areas  by  mountain  folding, 
or  by  the  weight  of  superincumbent  strata^  may  cause 
regional  metamorphism.  Obviously  such  changes,  espe- 
cially those  caused  by  heat,  result  in  the  complete  de- 
struction of  all  plant  or  animal  remains  or  impressions, 
and  thus  fossil  records  over  large  areas,  and  representing 
vast  periods  of  geologic  time,  have  been  obliterated. 

507.  Stratification  of  Rocks. — Changes  in  the  relative 
level  of  sea  and  land  have  occurred  many  times  in  the 
geological  past,  so  that  submerged  areas  of  sedimentation 
in  one  period  have  become  areas  of  dry  land,  undergoing 
erosion  in  another;  and  vice  versa,  areas  of  erosion  have 
become    areas    of    sedimentation.     As    a   result    of    this, 

^  Some  rocks  are  buried  under  more  than  40,000  feet  of  strata,  and  the 
temperature  increases  approximately  i°F.  for  every  50  to  60  feet  of  depth. 


PALEOBOTANY  583 

rocks  occur  in  layers/  the  deeper  lying  layers  (with  ex- 
ceptions readily  explained  by  geologists)  being  older  than 
those  above,  or  nearer  the  surface.  Moreover,  as  a  result 
of  a  second  submersion  following  elevation  and  erosion, 
subsequent  layers  were  often  deposited  with  an  uncon- 
formity on  the  weathered  and  eroded  surface  under- 
neath. 

By  the  presence  of  fossil  imprints  of  rain  drops,  foot- 
prints, ripple  marks,  and  mud  cracks,  and  by  the  character 
of  the  plant  and  animal  fossils  which  they  contain,  we 
know  that  most  sedimentary  rocks  were  deposited  in 
shallow  water,  not  far  from  the  shore  line.  But  since 
these  same  rocks  may  have  a  thickness  of  thousands  of 
feet  we  know  the  area  of  sedimentation  must  have  been 
slowly  sinking  while  the  sediment  was  being  deposited. 
As  a  result  of  the  enormous  pressure  of  the  overlying 
material,  of  the  deposit  of  cementing  substances  from 
solution,  and  of  other  causes,  the  sedimentary  deposits 
became,  in  time,  converted  into  solid  rock. 

508.  Classification  of  Rock  Strata.— By  a  study  of  the 
fossils  which  the  rocks  contain,  geologists  have  been  able 
to  classify  the  various  strata  according  to  their  age. 
As  a  result  of  the  period  of  erosion,  indicated  by  un- 
conformity, the  transition  from  the  stratum  of  one  age 
to  that  of  another  is  often  abrupt,  the  fossils  in  successive 
periods  being  quite  characteristic  of  the  given  stratum 
or  period.  In  other  cases,  as  for  example  between  the 
Silurian  and  Devonian  in  New  York  State,  there  is  no 
unconformity,  and  this  renders  it  more  difficult  to  decide 
just  where  the  plane  of  division  lies.  The  names  and 
order  of  occurrence  of  the  known  rock  strata  are  given  in 

^  Several  layers  form  a  stratum  or  bed. 


5^4 


STRUCTURE    AND    LIFE    HISTORIES 


the  following  table,  the  older  rocks  being  at  the  bottom, 
the  most  recently  formed  at  the  top. 


Table  VI. — Table  of  Geological  Time 


Era 


Cenozoic 


Mesozoic 


Quaternary 


Tertiary 


Secondary 


Paleozoic 


Primary 


Archean 


Period 
Holocene 

(recent,  or  the  present) 
Pleistocene 

(ice  age) 

Pliocene 
Miocene 
Oligocene 
Eocene 

Upper  Cretaceous 
Lower  Cretaceous 
(Comanchean) 
Jurassic 
Triassic 

Permian 

Upper  Carboniferous 

(Pennsylvanian) 
Lower  Carboniferous 

(Mississippian) 
Devonian 
Silurian 
Ordovician 
Cambrian 

/  Huronian 
\  Laurentian 


509.  Paleogeography. — By  changes  in  the  relative 
level  of  the  land  and  sea,  above  referred  to,  rocks  contain- 
ing fossils  may  be  elevated  as  dry  land,  and  frequently 
as  mountains,  so  that  remains  of  marine  organisms,  as 
well  as  of  others,  are  often  found  at  high  elevations.  In 
some  cases  forests  near  the  seashore  have  been  submerged, 


PALEOBOTANY 


sss 


Pjg    4J2. — Fossil  tree  stumps  lu  a  carboniferous  forest,  Victoria  Park, 
Glasgow.     (Cf.  Fig.  414-)     (After  Seward.) 


Fig.  414.— Part  of  a  submerged  forest  as  seen  at  low  tide  on  the  Cheshire 
coast  of  England.     (Cf.  Fig.  413.)     (After  Seward.) 


586  STRUCTURE    AND    LIFE    HISTORIES 

and  covered  over  with  sediment,  then  elevated  again  as 
dry  land,  so  that  subsequent  excavations  have  revealed 
the  fossilized  trunks  and  stumps  (Figs.  413  and  414).  Thus 
it  is  seen  that,  by  a  study  of  fossils,  we  may  not  only 
learn  of  their  structure  and  thus  fill  in  many  of  the  gaps 
in  the  evolutionary  sequence  left  by  a  study  of  forms  now 
living,  but  we  may  also  learn  of  the  distribution  of  plants 
and  animals  in  previous  geological  ages — in  other  words, 
we  have  the  basis  for  a  science  of  fossil  geography  or 
paleo  geography. 

610.  Plant  Migrations. — With  the  development  of 
Paleogeography,  a  clearer  conception  of  the  location  and 
changes  of  the  continental  areas  of  the  past  is  gradually 
being  gained.  As  a  consequence,  plant  geography  is  a  sub- 
ject of  increasing  interest  to  the  paleobotanist.  More- 
over, geology,  the  fossil  record,  and  the  present  zonal 
grouping  of  plants  indicate  that,  in  the  past,  the  polar 
areas,  once  tropic  or  sub-tropic,  must  have  been  fruitful  in 
new  species.^  High  mountains  or  plateaus  are  also  sug- 
gested as  homes  of  plastic  races.  In  the  tropics  environ- 
ments are  more  nearly  static,  and,  it  is  reasonable  to  sup- 
pose, less  Ukely  to  cause  variation.  It  is  known  that,  once 
established,  many  species  move  most  readily  along  the 
geologic  formation  which  supplies  the  exact  soil  constitu- 
ents, the  rate  of  movement  often  being  rapid.  Flotation 
of  seeds  is  also  a  factor.  The  facts  here  briefly  cited  rest 
on  the  observations  of  a  large  number  of  invesitgators, 
extending  over  more  than  a  century. 

511.  Distribution  of  Plants  in  Time.— In  addition  to 
the  distribution  of  plants  in  space  (plant  geography) ,  the 

1  Owing  to  the  precession  of  the  equinoxes  these  areas  undergo  an  extreme 
variation  in  the  length  of  winter  and  summer  of  37  days  every  1 2,934  years. 


PALEOBOTANY 


587 


problem  of  their  distribution  in  geologic  time  is  one  of 
absorbing  interest  and  importance.     The  following  table 
indicates  the  known  distribution  of   the  various  plant 
groups  from  the  earliest  geologic  time  to  the  present. 
Table  VII.— Distribution  of  Plants  in  Geologic  Time^ 


Division 

Subdivision,  class, 
or  order 

Range 

Common 
name  or 
example 

I 

2 

5j 

Dicotyledones 
Monocotyledones 

Comanchean  to  present 
Comanchean  to  present 

Oaks 
Grasses 

Spermato- 

< 

(Fossil  record  scant) 
Permian  to  present 
Permian  to  present 
Devonian  to  Permian 
Permian  to  present 
Devonian  to  Jurassic 

IV.  i  phyta 

Cycadophyta 

e 

i 
a 

0 

Gnetales 

Coniferales 

Ginkgoales 

Cordaitales 

Cycadales 

Cycadofilicales 

Ephedra 

Pines 

Ginkgo 

Cordaites 

Cycads 

Neuropteris 

^  Lepidophyta 

III.|  Calamophyta 

[  Pteridophyta 

Lycopodiales 

Equisitales 

Sphenophyllales 

Filicales 

Devonian  to  present 
Devonian  to  present 
Devonian  to  Permian 

Devonian  to  present 

Club  mosses 

Horsetails 

i  S  p  h  e  n  0  - 

phyllum 

Ferns 

II.  Bryophyta 

Musci 
Hepaticae 

Tertiary  to  present 
Tertiary  to  present 

1  Mosses 
1  Liverworts 

1 

I.  Thallophyta 

1 

Fungi 

Algae 

Diatomeae 

Schizophyta 

Myxomycetae 

Silurian  to  present 
Pre-Cambrian  to  present 
Jurassic  to  present 
Pennsylvanian  to  present 
(Fossil  record  lacking) 

Fungi 
Seaweeds 
Diatoms 
Bacteria 
1  Slime-molds 

1  Modified  from  Shimer. 


612.  Gaps  in  the  Fossil  Record. — In  the  Origin  of 
Species  Darwin  called  attention  to  the  paltry  display  of 
fossils  in  our  museums,  as  evidence  of  how  little  we  really 
know  of  the  plant  and  animal  life  of  past  ages,     "The 


588  STRUCTURE    AND    LIFE    HISTORIES 

number,  both  of  specimens  and  of  species,  preserved  in 
our  museums,''  says  Darwin,  ''is  absolutely  as  nothing 
compared  with  the  number  of  generations  which  must 
have  passed  away  during  a  single  formation."  The 
meagerness  of  the  record  is,  of  course,  due  in  part  to  the 
relatively  small  area  explored  in  proportion  to  the  whole; 
but  there  are  other  reasons  much  more  serious,  because 
they  represent  opportunities  lost  forever.  Among  them 
are  metamorphosis,  explained  above,  and  the  fact  that 
many  of  the  organisms  of  the  past  were  composed  wholly 
or  partly  of  soft  tissues,  which  were  entirely  destroyed,  by 
decay  or  otherwise,  in  the  process  of  rock-formation. 
Such  plants,  for  example,  as  Spirogyra  and  many  other 
algae,  the  fleshy  fungi,  and,  among  animals,  jelly-fish, 
earthworms,  and  others,  would  form  fossils  only  under 
exceptionally  favorable  circumstances,  if  at  all. 

But  there  is  an  even  more  effective  cause  of  oblitera- 
tion of  the  fossil  record  in  the  long-continued  erosion  and 
denudation  represented  by  unconformity  in  the  rock 
strata.  In  many  cases  only  a  small  proportion  now  re- 
mains of  the  thickness  of  a  rock  stratum  originally  de- 
posited, and  all  traces  of  the  plant  and  animal  life  that 
may  have  existed  on  the  denuded  area  have  thus  been  ob- 
literated forever.  These  blank  intervals  between  suc- 
cessive periods  were  of  vast  duration. 

"I  look  at  the  geological  record,"  said  Darwin,  ''as  a 
history  of  the  world  imperfectly  kept,  and  written  in  a 
changing  dialect;  of  this  history  we  possess  the  last 
volume  alone,  relating  only  to  two  or  three  countries. 
Of  this  volume,  only  here  and  there  a  short  chapter  has 
been  preserved;  and  of  each  page,  only  here  and  there  a 
few  lines.     Each  word  of  the  slowly  changing  language, 


PALEOBOTANY  589 

more  or  less  different  in  the  successive  chapters,  may 
represent  the  forms  of  life,  which  are  entombed  in  our 
consecutive  formations,  and  which  falsely  appear  to  have 
been  abruptly  introduced."^  These  views  have  received 
added  emphasis  from  the  recent  development  of  Paleo- 
geography. 

513.  Factors  of  Extinction. — The  question  may  natu- 
rally arise,  ''Why  did  the  species  common  in  previous  geo- 
logical ages  die  out,  giving  place  to  newer  forms?"  The 
answer  is  found  in  the  facts  of  struggle  for  existence  and 
survival  of  the  fittest.  In  the  words  of  the  great  American 
botanist,  Asa  Gray,  species  may  continue  only  ''while 
the  external  conditions  of  their  being  or  well-being  con- 
tinue." The  struggle  may  be  with  other  organisms  or 
with  the  physical  conditions  of  the  environment.  Among 
the  more  important  factors  of  extinction,  may  be  men- 
tioned the  following: 

1.  Struggle  with  other  plants  for  adequate  space.  This  is 
illustrated  in  a  simple  way  by  the  crowding  out  of  cultivated 
plants  by  weeds  in  a  garden.  By  more  rapid  germina- 
tion and  growth,  and  by  other  "weedy"  characteristics, 
the  weeds  get  the  start  of  the  cultivated  plants,  occupying 
all  available  space,  and  choking  them  out. 

2.  Attacks  of  disease-causing  parasites,  e.g.,  chestnut 
trees  by  a  parasitic  fungus,  elm  trees  by  the  elm  tree  beetle. 

3.  Changes  of  environment  too  great  or  too  rapid  to  permit 
oj  readjustment.  Plants  are  plastic  organisms,  and  can 
adapt  or  readjust  themselves  to  considerable  environ- 
mental change,  but  there  are  limits  of  speed  and  amount 
of  change  beyond  which  readjustment  is  not  possible,  and 
the  plant  must   consequently   perish.     If   such   changes 

1  Darwin,  C.     '^Origin  of  Species,"  New  York,  1902,  vol.  2,  p.  88. 


590  STRUCTURE    AND    LIFE    HISTORIES 

involve  the  entire  area  of  distribution  of  the  species  con- 
cerned, the  species  will,  obviously,  become  extinct.  The 
following  seven  factors  are  specific  instances  of  this. 

4.  Diminished  water  supply.  Aquatic  plants  may  be 
destroyed  by  the  draining  of  a  pond  or  lake;  hydrophytic 
forms  by  the  drying  up  of  a  swamp.  Sometimes  forms 
suited  to  conditions  of  moderate  water  supply  {hydro- 
phytes) are  destroyed  by  the  conversion  of  wide  areas  into 
desert  regions,  as  has  doubtless  occurred.  If  such  changes 
are  gradual,  resting  spores  {e.g.,  Spirogyra),  winter  buds 
{e.g.,  Utricularia,  and  eel-grass),  and  seeds  readily  trans- 
ported by  wind  {e.g.,  cat-tail)  enable  the  species  to  become 
reestablished  in  a  new  location,  but  not  so  when  the 
changes  are  too  abrupt,  or  cover  too  wide  an  area. 

5.  Temperature  changes,  when  too  abrupt,  too  extreme, 
or  too  long  continued.  When  the  continental  ice-sheet 
advanced  southward  during  the  glacial  period,  many 
forms,  adapted  only  to  temperate  conditions,  became  ex- 
tinct. Fossils  of  extinct  tropical  plants  are  found  in 
Greenland,  which  is  now  undergoing  a  glacial  period. 

6.  Volcanic  eruptio7is,  such,  for  example,  as  those  of 
Mount  Pelee,  which  occurred  in  1902,  on  the  island  of 
Martinique,  W.  I.,  often  destroy  all  signs  of  life  over  a 
radius  of  many  miles.  In  the  states  of  Washington, 
Oregon,  and  Idaho  floods  of  molten  lava,  covering  thous- 
ands of  square  miles,  have  been  poured  out  over  the  sur- 
face, forming  a  wide  plateau.  It  is  almost  certain  that 
many  species  of  plants  and  animals  have  become  extinct 
by  such  agencies.  Not  only  the  lava,  but  poisonous 
gases  that  fill  the  air  during  volcanic  eruptions,  are  fatal 
to  plant  life. 

7.  Encroachment  of  salt  water  in  coastal  regions,  caused 


PALEOBOTANY  59 I 

by  changes  in  the  level  of  the  land,  resulting  in  the  killing 
of  fresh-water  vegetation. 

8.  Disturbance  of  symbiotic  relationships.  The  inter- 
relationships of  organisms  are  very  complex,  affording 
innumerable  opportunities  for  extinction  by  a  disturbance 
of  adjustments.  Shade-loving  forms  in  a  forest  may 
perish  by  the  destruction  of  those  affording  the  shade; 
obligate  parasites  may  perish  from  the  destruction  of  the 
necessary  host;  plants  dependent  upon  certain  insects  for 
cross-pollination  may  perish  on  account  of  the  extinction 
of  the  necessary  insects. 

9.  Diminution  of  carbon  dioxide  in  the  atmosphere.  There 
are  reasons  for  thinking  that  in  certain  past  ages  the  at- 
mosphere was  richer  than  now  in  carbon  dioxide,  and  that 
that  condition  was  favorable  to  the  development  of  certain 
vegetatively  vigorous  species  which  cannot  live  in  an 
atmosphere  like  the  present,  having  a  smaller  percentage 
of  carbon. 

10.  Denudation  of  the  land  surface.  In  the  course  of 
ages  even  lofty  mountains  are  planed  down  by  erosion, 
and  the  arctic  and  sub-arctic  species  of  the  high  altitudes 
thus  undergo  extinction.  Furthermore,  erosion  may  be 
coupled  with  general  subsidence.  In  fact,  not  only  do 
geologists  now  recognize  numerous  old  mountain  '  roots," 
such  for  example  as  the  Adirondack  region  of  New  York 
State,  but  there  are  also  abundant  evidences  of  periodic 
emergence  and  subsidence  of  areas  of  continental  extent, 
quite  throughout  geologic  time.  The  climatic  and  other 
environmental  disturbances  accompanying  such  changes 
would  inevitably  result  in  the  extinction  of  certain  species- 
(See  also  ^[505.) 


CHAPTER  XXXVIII 
THE  EVOLUTION  OF  PLANTS  (Concluded) 

514.  Evidences  from  Fossil  Plants. — The  study  of  fossil 
plant  remains  has  greatly  enlarged  our  knowledge  of 
the  course  of  plant  evolution,  filling  in  gaps  derived  from 
the  study  of  living  forms,  and  affording  new  facts,  not 
disclosed  by  the  study  of  plants  now  living.  Like  the 
study  of  comparative  anatomy  and  life  histories,  paleo- 
botany teaches  us  that  there  has  been  a  gradual  evolu- 
tionary progress  from  the  simple  to  the  more  complex,  but 
it  has  also  disclosed  the  fact  that  some  of  the  complex 
forms  are  much  more  ancient  than  had  been  inferred  from 
the  study  of  living  plants  only. 

515.  Discovery  of  Seed-bearing  Ferns. — For  example, 
remains  of  seed-bearing  plants,  quite  as  highly  organized 
as  those  of  to-day,  are  found  far  back  in  the  earliest  fossil- 
bearing  strata  of  the  Paleozoic.  Great  forest  types  ex- 
isted as  early  as  the  Devonian.  Later  in  the  Carboniferous 
occur  many  seed-bearing  ferns.  These  have  been 
called  Cycadofilicales  (cycadaceous  ferns),  or,  by  some, 
Pterido sperms.  Recent  studies  have  disclosed  the  fact 
that  most  of  the  fossil  plants  from  the  Carboniferous  coal- 
bearing  strata,  formerly  thought  to  be  ferns,  are  not  even 
cryptogams,  but  are  these  fern-like  seed-bearing  plants. 
The  best  known  pteridosperm  is  Lyginodendron  oldhamium 
(Fig.  415),  first  described  from  fossil  leaves,  in  1829,  as 
a  tree-fern,  under  the  name  Sphenopteris  Eoeninghausi. 

592 


THE  EVOLUTION  OF  PLANTS 


593 


After  investigations  extending  over  nearly  90  years,  ''we 
are  now  in  position  to  draw  a  fairly  complete  picture  of 
the  plant  as  it  must  haVe  appeared  when  living. 

"It  was  in  effect  a  little  tree-fern,  with  long,  slender, 


Fig.  415, — Lyginodendron  oldhamium.  Pinna  of  a  microsporophyll, 
found  in  an  ironstone  nodule.  Before  its  identity  was  established  this 
specimen  was  named  Crossotheca  Hoeni7ighausi.  The  somewhat  pellate 
fertile  pinules  on  the  ultimate  branches,  bear  each  a  fringe  of  micro- 
sporangia  about  3  mm.  long.  The  appearance  has  been  likened  to  that 
of  a  fringed  epaulet.     (After  Scott,  from  a  photo  by  Kidston.) 


sometimes  branched,  stem,  4  centimeters  or  less  in  diame- 
ter, and  provided  with  spines  by  means  of  which  it 
probably  climbed  on  its  neighbors.  The  foliage  was  dis- 
posed spirally  and  consisted  of  relatively  very  large,  finely 


^ 


594 


STRUCTURE    AND    LIFE    HISTORIES 


divided  fronds  with  small,  thick  pinnules  with  revolute 
margins,  suggesting  a  xerophytic  or  halophytic  habitat. 
The  stem  in  the  lower  portion  gave  rise  to  numbers 
of  slender  roots,  some  of  which  appear  to  have  been 
aerial  in  their  origin.  These  grew  downward  and  often 
branched  where  they  entered  the  soil. 


Fig.  416. — Young  leaf  of  the  Cycad,  Bowejiia  serrulata.  Comparison 
of  this  with  a  leaf  of  the  fern  Angiopteris  (Fig.  417)  shows  how  difficult 
it  might  be  to  decide  from  a  fossil  leaf  whether  the  plant  was  a  cycad  or  a 
fern.     (Cf.,  also,  Fig.  420.) 

''The  stems,  roots,  and  petioles,  and  even  the  pinnules, 
have  been  found  [calcified]  and  so  beautifully  preserved  that 
their  entire  structure  can  be  made  out  with  certainty. 
Without  going  into  a  technical  description  of  these  organs, 
it  may  be  said  that  the  stem  when  young,  and  before 
secondary  growth  has  begun,  has  a  very  strong  resemblance 


THE  EVOLUTION  OF  PLANTS 


595 


to  the  stem  of  [the  fern]  Osmunda,  but  when  more  mature 
certain  cycadean  characters  appear  to  predominate."^ 


Fig.  417. — ^Leaf  of  a  fern  (Angiopkris  cvecta).     (Cf.  Fig.  416.) 


Its  foHage  and  other  characters  closely  resemble  some 
of  our  modern  tree-ferns  (Cf.  Figs.  416  and  417)  but  more 
careful  study  of  the  calcified  specimens  of  much  beauty 

^  Knowlton,  F.  H.     American  Fern  Journal,  5  :85.     1915. 


596 


STRUCTURE    AND    LIFE    HISTORIES 


found  in  the  English  "coal  balls '^^  has  disclosed  both  the 
microsporophylls,  bearing  pollen-sacs,  and  the  megasporo- 
phylls,  bearing,  not  merely  megasporangia,  but  true  seeds. 
The  ovule  has  a  pollen-chamber,  like  the  cycads,  except  that 
it  projects  a  bit  through  the  micropyle,  and,  strange  as  it 
may  seem,  fossil  pollen-grains  have  been  discovered,  well 
preserved  within  this  chamber.  The  seeds,  about  3^  inch 
long,  have  been  described  as  resembling  little  acorns,  en- 


FiG.  418. — Restoration  of  a  seed  of  Lyginodendro7i  oldhaviium  {Lagenos- 
tema  Lomaxi),  from  a  model  by  H.  E,  Smedley.     (After  Scott.) 


closed  like  hazelnuts  in  smaller  glandular  cupules  (Fig. 
418).  They  are  similar  to  those  of  the  cycads,  except 
that  they  are  not  known  to  have  organized  an  embryo  with 
cotyledons  and  caulicle.  Instead,  the  tissues  of  the 
female  gametophyte  only  are  so  far  found,  retained  within 

^  Coal  balls  are  "concretions  of  the  carbonates  of  lime  and  magnesia 
which  formed  around  certain  masses  of  the  peaty  vegetation  as  centers 
and,  through  inclosing  and  interpenetrating  them,  preserved  them  from 
the  peculiar  processes  of  decay  which  converted  the  rest  of  the  vegetation 
into  coal.  In  them  the  mineral  matter  slowly  replaced  the  vegetable 
matter,  molecule  by  molecule,  thus  preserving  the  cellular  structure  to  a 
remarkable  degree.  Such  balls  are  especially  frequent  in  the  coal  of 
certain  parts  of  England  (Lancashire  and  Yorkshire)." 


THE    EVOLUTION    OF   PLANTS 


597 


the  megasporangium,  which  is  enclosed  in  the  integument. 
In  this  connection  it  is  of  interest  to  note  that  the  seeds  of 
some  modern  plants  {e.g.,  orchids)  do  not  possess  differ- 
entiated embryos,  but  whether  this  is  a  primitive  or  a 
reduced  character  is  not  certain.  The  pollen  was  formed 
in  spindle-shaped  pollen-sacs,  having  two  chambers  and 
borne  in  clusters  of  four  to  six  on  the  under  side  of  little 
oval  discs  from  2  to  3  millimeters  long.     These  structures 


Fig.  419. — Top, lateral  pinna  from  a  leaf  of  Marattia  Jraxinea.  (After 
Bitter.)  Below  at  left,  synangium  of  same.  (After  Bitter.)  At  right, 
cross-section  of  the  synangium.     (After  Hooker-Baker.) 


are  found  on  pinnules  of  ordinary  foliage  leaves,  resem.bUng 
the  sporophylls  of  certain  ferns  (Fig.  419)  rather  than  the 
stamens  of  modern  flowers. 

The  discovery  of  the  seed-bearing  character  of  the 
fern-like  plants  of  the  Paleozoic  was  predicted  by 
Wieland,  of  Yale  University,  nearly  two  years  before  it 
was  made  by  Oliver  and  Scott.  It  is  now  believed  that 
seed-bearing  plants  of  the  pteridosperm  type  were  nearly 
as  numerous  in  the  Paleozoic  as  were  the  cryptogams. 


598 


STRUCTURE    AND    LIFE   HISTORIES 


516.  Significance  of  the  "Pteridosperms." — The  close 
resemblance  of  the  pteridosperms  to  ferns,  on  the  one  hand, 
and  to  modern  cycads  on  the  other,  justifies  the  conclu- 
sion that  they  represent  a  "connecting  link"  between  the 
true  ferns  and  the  cycads,  and  that  the  modern  cycads 
have  descended  from  the  same  ancestry  as  the  modern 
ferns,   each   developing  along  somewhat  different  lines. 


Fig.  420. — Stangeria  paradoxa  JMoore.  Specimen  from  the  cycad 
house  at  the  New  York  Botanical  Garden,  bearing,  at  the  apex  of  the  stem 
a  carpellate  cone.     (Photo  from  New  York  Botanical  Garden.) 

It  was  in  recognition  of  their  vegetative  resemblances  that 
the  Pteridosperms  were  first  called  (by  Potonie)  Cycado- 
filices,  now  Cycadofilicales.  Van  Tieghem  tersely  de- 
scribed them  as  "phanerogams  without  flowers." 

517.  A  Modern  Fern-like  Cycad. — One  of  the  modern 
cycads  {Stangeria  paradoxa)^  is  of  much  interest  in  this 

^  Stangeria  paradoxa  Moore  =  Stangeria  eriopus  (Kunze)  Nash. 


THE  EVOLUTION  OF  PLANTS  599 

connection.  So  closely  does  it  resemble  a  certain  fern 
{Lomaria)  that  the  botanist  Kunze,  who  first  described  it 
when  it  was  brought  from  Natal  to  the  botanic  garden  at 
Chelsea,  England,  supposed  it  was  a  fern,  and  named  it 
Lomaria  eriopus.  The  specimen  possessed  no  fruit,  which 
would  have  helped  to  identify  it.  Its  leaves,  with 
circinate  vernation,  have  a  pinnately  compound  blade, 
and  leaflets  with  pinnate  dichotomous  venation.  Two 
or  three  years  later  another  botanist,  examining  it  more 
closely,  pronounced  it  a  "fern-like  Zamia  or  a  Zamia-like 
fern."  These  facts  show  how  puzzling  the  specimen  was, 
and  how  closely  a  plant  may  resemble  both  a  cycadophyte 
and  a  fern.  In  a  sense  this  plant  may  be  called  a  living 
fossil.  Specimens  have  since  come  into  flower  in  botanic 
gardens,  and  the  typical  cycadaceous  cones  (Fig.  420) 
leave  no  doubt  that  the  plant  is  a  true  cycadophyte. 

618.  Derivation  of  New  Types. — Attention  should  here 
again  be  called  to  the  fact  that  the  theory  of  evolution  does 
not  teach  that  one  given  species  becomes  transformed  into 
another,  but  simply  that  new  species  are  descended  from 
older  forms  which  may  or  may  not  continue  to  exist.  It 
is  not  supposed,  for  example,  that  ferns  developed  into 
cycads,  and  cycads  into  higher  gymnosperms,  but  that  there 
has  been  an  unbroken  line  of  descent  (possibly  more  than 
one)  in  the  plant  kingdom ;  that  closely  related  forms  (like 
ferns  and  cycads)  have  descended  from  a  common  ancestral 
type  which  may  or  may  not  now  be  found.  We  must  not, 
in  other  words,  expect  necessarily  to  find  in  fossil  forms  the 
direct  ancestors  of  those  now  living,  although  a  study  of 
their  structure  is  of  the  greatest  value  in  enabling  us  to 
understand  the  genetic  relationships  of  the  great  groups 
of  plants. 


6oo 


STRUCTURE    AND    LIFE    HISTORIES 


619.  Ancestors    of    the    Angiosperms.— Just    as    the 

Cycadofilicales  indicate  the  ancestry  of  the  cycads,  so 
fossil  types  of  Cycadophyta  have  been  discovered  which 
are  interpreted  by  some  paleobotanists  as  ancestors  of  the 
modern  angiosperms.  Other  investigators,  however,  dis- 
sent from  this  view  and  consider  that  we  have  not  yet 


Fig.  421. — To  the  left,  Cycadeoidea  dacotensis  Macbride.  Longitudinal 
section  of  a  silicified  specimen  of  a  bisporangiate  cone  (unexpanded  flower), 
so  taken  that  the  pinnules  of  the  microsporophylls  on  both  sides  of  the 
central  axis,  or  receptacle,  are  successively  cut  throughout  their  entire 
length.  The  lines  indicate  the  planes  of  various  sections  through  the  cone, 
published  in  Wieland's  "American  Fossil  Cycads."  To  the  right  Cycado- 
cephalus  Sewardi  Nathorst.  Microsporangiate  cone,  natural  size,  preserved 
as  an  impression  on  a  flat  slab.  From  a  fossil-bearing  bed  of  the  Trias,  at 
Bjuf,  Southern  Sweden.  (Left  figure  from  Wieland,  right  figure  from 
Nathorst.) 

sufficient  knowledge  of  fossil  forms  to  be  justified  in 
designating  the  ancestors  of  the  Angiosperms.  This 
difference  of  opinion  is  largely  due  to  the  meagerness  of 
the  available  evidence.  As  one  writer  has  stated  it, 
''A  trayful  of  flowers  may  be  all  the  record  of  the  Pterido- 


THE  EVOLUTION  OF  PLANTS  6oi 

sperms  from  the  Devonian  on.     The  gaps  in  the  evidence 
are  always  enormous." 

Although  the  Cycadophyta  are  now  a  very  insignificant 
element  in  the  earth's  flora;  in  the  Mesozoic  period  they 
form  about  one-third  of  the  recovered  vegetation  of  the 
land.     One  order,  the  Bennetti tales,  then  had  a  cosmo- 


FiG.  422. — Cycadeoidea  dacotensis.  Semi-diagrammatic  sketch  of  a 
flower  (bisporangiate  cone),  cut  longitudinally;  one  sporophyll  folded,  and 
one  (at  the  right)  arbitrarily  expanded.  At  the  center  is  the  apical,  cone- 
shaped  receptacle,  invested  by  a  zone  of  short-stalked  ovules  and  inter- 
seminal  scales.  The  pinnules  of  the  sporophylls  bear  the  compound 
sporangia  {Synangia).  Exterior  to  the  flower  are  several  hairy  bracts. 
About  three-fourths  natural  size.     (After  Wieland.) 

politan  distribution  and  seemingly  was  as  important  as 
the  Dicotyledons  are  now.  Over  30  species  of  the  petrified 
stems  have  been  found  in  the  Mesozoic  terrains  of  the 
United  States,  the  Black  Hills  of  South  Dakota  alone 
yielding  a  score.     The  Isle  of  Portland  forms  were  called 


6o2  STRUCTURE    AND    LIFE    HISTORIES 

Cycadeoidea  by  the  celebrated  geologist  Buckland.  The 
name  of  the  order  is  derived  from  the  genus-name,  Bennet- 
tites}  Other  forms,  usually  found  as  casts,  are  called 
Williams onia,  still  others  are  known  mainly  as  genera 
founded  on  leaf  imprints. 

520.  Cycadeoidea. — In  most  of  its  purely  vegetative 
characters,   such  as  the  anatomy  of  the  stem  and  the 


Fig.  423. — Cycadeoidea  dacolcnsis  (?).  Photomicrograph  of  a  young 
seed  (X  15),  showing  a  sterile  scale  on  either  side.  Between  them  pro- 
jects the  entire  length  of  the  tube  through  which  the  micropyle  extends. 
The  partially  collapsed  nucellus  is  distinctly  shown  in  the  center.  (After 
Wieland.) 

structure  of  the  leaves,  Cycadeoidea  resembled  modern 
cycads,  but  its  reproductive  branches  were  character- 
istically lateral,  which  is  one  of  the  most  fundamental 
characteristics  of  the  higher  seed-bearing  plants  of  to- 
day. Only  two  modern  cycads  {Macrozamia  and  Bowenia) 
have    lateral    seed-bearing    cones    (Fig.    289).^     Various 

^  Cycadeoidea  Buckland  =  Bennellites  Carruthers. 
2  The  staminate  cones  of  Zamia  are  lateral. 


THE   EVOLUTION   OF   PLANTS 


603 


Fig.  424.—Cycadoidea  Wielandi.  At  left,  a  finely  preserved  trunk 
bearing  many  ovulate  cones  with  seeds  approaching  maturity,  and  a  lesser 
number  of  either  young  or  abortive  cones.  /',  Receptacle  of  a  shed  or 
non-preserved  cone  with  surrounding  bracts  3^et  present;  /' ,  two  cones 
broken  away  during  erosion,  with  a  portion  of  the  basal  mfertile  pedicel 
yet  remaining;  m,  four  cones  eroded  down  to  the  surface  of  the  armor, 
in  this  instance  about  or  a  little  beneath  the  level  of  the  lowermost 
seeds;  y,  three  of  the  dozen  or  more  very  young  cones,  in  some  cases  known 
to  be' simply  ovulate  and  to  be  regarded  as  having  aborted  or  else  as  be- 
longing to  a  later  and  sparser  series  of  fructifications  than  the  seed-bearing 
cones  present,  the  latter  unquestionably  representing  the  culminant  fruit- 
producing  period  in  the  life  of  this  cycad;  5  (over  lower  arrow),  the  ovulate 
strobilus  shown  at  right  in  its  natural  position,  this  photograph  having 
been  made  before  the  cone  was  cut  out  by  a  cylindrical  drill.  X  about 
14  At  right,  longitudinal  section  of  the  small  ovulate  strobilus  cut  from 
its  natural  position  on  the  trunk  as  denoted  by  the  arrow  s,  in  photograph 
I  c  (upper  arrow),  seed  with  dicotyledonous  embryo  preserved,  cotyle- 
dons being  similarly  present  in  the  lowermost  seed  on  the  left-hand  side 
of  the  strobilus;  s,  traces  of  hypogynous  staminate  disk;  b,  bracts;  /,  leaf 
bases.     X  about  K-     (After  Wieland.) 


6o4 


STRUCTURE    AND    LIFE    HISTORIES 


structural  characters  of  Cycadeoidea  are  shown  in  Figs. 
421-427. 

In  Cycadeoidea  dacotensis  the  ''flower,"  which  in  some 
specimens  was  5  inches  long,  was  a  strobilus,  consisting  of 
a  thick  axis  on  the  lower  part  of  which  were  numerous 
bracts   arranged   in   spirals.     The   bracts   surrounded   a 


P'v 


m^^mmmm: 


Fig.  425. — Cycadeoidea  Wielandi.  Longitudinal  section  through  the 
axis  of  a  female  inflorescence,  or  cone.  Z,  old  leaf-base;  d,  insertion  of 
disc;  s,  erect  seed,  borne  at  summit  of  seed-pedicle  inserted  on  convex 
receptacle;  b,  hair-covered  bract.     (After  Wieland.) 

campanula  of  about  20  stamens.  Each  stamen  was,  in 
reality,  a  pinnately  compound  sporophyll,  about  4  inches 
long,  rolled  in  toward  the  center  of  the  flower,  and 
bearing  two  rows  of  compound  microsporangia  (pollen- 
sacs)  on  each  leaflet.  They  thus  closely  resembled  the 
sporophyll  of  a  fern. 


THE   EVOLUTION  OF  PLANTS 


60s 


Fig.  426. — Cycadeoidea  ingens.  Restoration  of  an  expanded  bispor- 
angiate  cone,  or  flower,  in  nearly  longitudinal  section.  Restored  from  a 
silicified  fossil.     (After  Wieland.) 


' '  M 

■5 

^^BM^ 

^^^mm 

i^.    •       * 

fti^^ 

Sl%  V.  ■ 

K.< 

^^^ 

pl^^^ 

P^r'"^-'^^^' 

:  ^''^ 

^m»M 

H 

ij.-r'[' 

&. 

m 

,__-      .-^   .         .  . 

.  jt 

^  Fig.  427. — Cycadeoidea  Dartoni,  Tangential  section  through  outer 
tissues  of  the  (fossilized)  trunk,  showing  the  very  numerous  seed-cones. 
The  seeds  are  very  small  (the  illustration  being  natural  size),  and  nearly 
every  one  has  a  dicotyledonous  embryo.  There  were  over  500  such  cones 
on  the  original  stem.     (After  a  photograph  loaned  by  Prof.  Wieland.) 


6o6  STRUCTURE    AND    LIFE    HISTORIES 

The  axis  of  the  flower  terminated  in  a  cone-shaped 
receptacle,  bearing  the  stalked  ovules,  and  numerous 
sterile  scales  (Figs.  424  and  425).  The  mature  seeds  often 
contain  the  well-preserved  fossil  embryos,  with  two 
cotyledons  which  quite  fill  out  the  nucellus,  and  show 
that  there  was  little  or  no  endosperm.  These  are  char- 
acters never  found  in  the  lowest  group  of  modern  seed- 


FiG.  428. — Flower  of  magnolia.     (Cf.  Fig.  429.) 

bearing  plants  (the  Gymnosperms),  but  only  in  the 
highest  group  of  Angiosperms,  the  Dicotyledons.  In 
fact,  the  French  paleobotanist,  Saporta,  called  some  of  the 
Cycadeoids,  Proangios perms. 

521.  Relation  of  Cycadeoidea  to  Modem  Angiosperms. 
— The  question  of  the  ancestry  of  the  Angiosperms  is  the 
most  important  problem  of  paleobotany.  Although  the 
Bennetti tales  possess  many  of  the  primitive  anatomical 


THE  EVOLUTION  OF  PLANTS 


607 


features  that  characterize  the  Cycadofilicales,  their 
development  of  a  bisporangiate  strobilus  with  two  set 
of  sporophylls,  related  to  one  an- 
other as  they  are  in  the  flower  of 
the  Angiosperms.  indicates  a  gen- 
etic relationship  to  that  group,  as 
does  also  the  fact  that  the  seeds, 
enclosed  in  a  fruit,  possess  a  dicot- 
yledonous embryo,  without  endo- 
sperm. In  other  features  the  Ben- 
nettitales  are  unlike  the  Angio- 
sperms; the  ovules,  for  example, 
are  enclosed  by  sterile  scales,  in- 
stead of  by  the  carpels  on  which 
they  are  borne,  and  the  protrusion 
of  the  pollen-chamber  through  the 
micropyle  signifies  the  gymno- 
spermous  type  of  fertilization. 

These  and  other  comparisons  in- 
dicate that  the  Bennettitales  were 
essentially  Gymnosperms  having 
certain  Angiospermous  characters, 
and  therefore,  while  they  are  not 
to  be  considered  as  the  ancestors 
of  the  Angiosperms,  it  is  probable 
that  they  and  the  modern  dicoty- 
ledons are  both  descended  from  a 
common  branch  of  the  ancestral 
tree.  Among  modern  plants,  the 
flower  of  the  magnolias  most 
closely  resembles  that  of  Cycadeoidea  in  the  spiral  arrange- 
ment of  its  stamens  and  pistils  (Figs.  428  and  429.).     How 


Fig.  429. — Magnolia. 
Flower  with  perianth  re- 
moved, showing  the  com- 
pound pistil,  and  four  of  the 
stamens.  Most  of  the  sta- 
mens have  been  removed. 
Note  their  spiral  arrange- 
ment as  shown  by  the  scars 
at  the  points  of  attachment. 
(Cf.  Fig.  428.) 


6o8 


STRUCTURE    AND    LIFE    HISTORIES 


much  significance  should  be   attached  to  that  fact  has 
been  disputed  by  students  of  morphology. 

The  gap  between  the  stamen  of  Cycadeoidea  and  the 
type  characteristic  of  modern  Angiosperms  is  partially 
bridged  by  the  genus  Williamsonia  (which  has  simple 
vs.  pinnately  compound  stamens),  and  by  another  genus, 
Wielandiella,  both  older  genera  than  Cycadeoidea.  From 
this  it  has  been  inferred  that  the  Bennet- 
titales  are  a  lateral  branch,  further  re- 
moved than  their  ancestors  from  the  direct 
evolutionary  stock  of  the  Angiosperms. 
522.  Origin  of  Monocotyledons. — If 
the  earliest  Angiosperms  were  dicotyle- 
dons, as  seems  to  be  the  case,  the  mono- 
cotyledons were  probably  derived  from 
the  dicotyledons  by  a  process  of  simplifi- 
cation. Much  light  has  been  thrown  on 
this  question  by  a  study  of  the  develop- 
ment of  the  embryos  {emhryogeny)  of 
certain  plants.  The  case  of  Agapanthus 
umbellatus  L'Her.  (Fig.  430),  a  South 
African  plant  of  the  Lily  family,  may 
be  taken  as  illustrating  the  nature  of 
the  evidence  derived  from  embryogeny. 
The  sequence  of  events  is  as  follows.^  As  the  mas- 
sive proembryo  enlarges,  the  root-end  elongates,  thus  re- 
maining narrow  and  pointed;  while  the  shoot-end  widens, 
becoming  relatively  broad  and  flattish.  At  this  broad 
and  flat  end  the  peripheral  cells  remain  in  a  state  of 
more  active  division  than  do  the  central  cells,  and  form 
what  is  known  as  the  cotyledonary  zone.     In  this  zone  two 

1  The  above  description  closely  follows  Coulter  and  Land  (1914). 


Fig.  430. — Aga- 
panthus umhellatiis. 
A,  monocotyledon- 
ous  embryo;  5, 
dicotyledonous  em- 
bryo. (Redrawn 
from  photo  by  W. 
J.  G.  Land.) 


THE   EVOLUTION   OF   PLANTS  609 

more  active  points  (primordia)  appear  and  begin  to  de- 
velop. Soon  the  whole  zone  is  involved  in  more  rapid 
growth,  resulting  in  a  ring  or  tube,  but  with  the  primordia 
still  evident.  The  cotyledonary  zone  continues  its  growth 
until  a  tube  of  considerable  length  is  developed,  leaving 
the  apex  of  the  proembryo  depressed.  At  this  stage  either 
one  of  two  things  may  occur.  As  the  cotyledonary  zone 
continues  to  grow,  the  two  primordia  on  the  rim  of  the 
tube  may  continue  to  develop  equally,  forming  two  coty- 
ledons; or  one  of  the  primordia  may  cease  to  grow,  result- 
ing in  an  embryo  of  only  one  cotyledon;  in  other  words, 
the  entire  cotyledonary  zone  may  develop  under  the 
guidance  of  only  one  growing  point.  One  cotyledon  is 
not  eliminated,  but  the  whole  growth  is  diverted  into  one 
cotyledon.  There  thus  develops  what  appears  to  be  an 
''open  sheath"  and  a  ''terminal"  cotyledon. 

In  other  words,  monocotyledony  is  not  the  result  of  the 
fusion  of  two  cotyledons,  nor  of  the  suppression  of  one; 
but  is  simply  the  continuation  of  one  growing  point  on  the 
cotyledonary  ring,  rather  than  a  division  of  the  growth  be- 
tween two  growing  points.  In  a  similar  way,  polycoty- 
ledony  is  the  appearance  and  continued  development  of 
more  than  two  growing  points  on  the  cotyledonous  ring. 
The  rudimentary  second  cotyledon  of  a  "monocotyle- 
donous"  grass-embryo  (wheat)  is  shown  in  Fig.  378,  (p.  494). 

523.  Ancestors  of  the  Gymnospenns. — As  far  back  as 
Devonian  time,  preceding  the  great  coal  period  (Carbon- 
iferous), fossils  have  been  found  of  a  plant,  Cordaites  (of 
the  order  Cordaitales) ,  common  in  that  period,  and 
having  characters  which  indicate  that  it  stands  in  the 
ancestral  line  of  our  modern  conifers — that  it  and  the 
conifers  had  a  common  ancestry. 
39 


6io 


STRUCTURE    AND    LIFE    HISTORIES 


The  leaves  of  Cordaites  resembled  those  of  the  Kauri 
pines  (Agathis)  of  the  southern  hemisphere  (Fig.  431), 
or  the  leaflets  of  Zamia.  They  varied  from  a  decimeter  to 
over  a  meter  in  length.  The  male  cones  resembled  those 
of  the  still  living  Ginkgo^  each  stamen  having  from  four 
to  six  microsporangia  (pollen-sacs)  on  a  stalk.     The  female 


Fig.  431. — Branch,   with   cones,   of   the  Kauri  pine  {Agalhis  australis). 
(From  Gardener's  Chronicle.) 

cones  resembled  the  male  in  general  appearance,  and 
the  seeds  resembled  those  of  the  Cycadofilicales  (Fig.  423). 
The  plant  itself  was  a  slender  tree,  some  forms  of  which 
attained  a  height  of  over  100  feet.  The  Cordaitales 
formed  the  world's  first  great  forests.  They  represent  a 
wide  departure  from  the  Cryptogams,  and  must  be  con- 
sidered as  true  seed-bearing  plants.     They  were  closely 


THE  EVOLUTION  OF  PLANTS  6ll 

related  to  the  Ginkgo — another  living  fossil,  ranking  next 
below  the  modern  cone-bearing  trees.  We  thus  ascend 
from  the  ferns  to  the  conifers  by  a  series  of  transitional 
forms  as  follows  (reading  from  the  bottom,  up): 

6.  Coniferales  (modern  cone-bearing  trees). 

•5.  Ginkgoales  (primitive  gymnosperms). 

4.  Cordaitales  (transitional  conifers). 

3.  Cycadales  (true  cycads). 

2.  Cycadofilicales  (cy cad-like  ferns). 

I.  Filicales  (true  ferns). 

524.  Relation  of  the  Above  Groups.— It  must  not  be 
inferred  that  the  above  groups  were  derived  one  from  the 
other  by  descent  from  lower  to  higher.  They  should  be 
interpreted  rather  as  samples  remaining  to  show  us,  not 
the  steps,  but  the  kinds  of  steps  through  which  the  plant 
kingdom  has  passed  in  developing  the  more  highly  organ- 
ized, modern  cone-bearing  trees  from  more  primitive  forms 
like  the  ferns.  As  stated  above,  it  is  doubtful  if  the  actual 
transitional  forms  have  been  preserved,  so  that  the  entire 
history  of  development  can  probably  never  be  written. 

525.  A  Late  Paleozoic  Landscape.— Fig.  432  illustrates 
the  kind  of  landscape  that  must  have  been  common  in  the 
latter  part  of  the  Paleozoic  era  along  sluggish  streams  in 
certain  regions  such  as  Texas  and  New  Mexico.  Of  the 
primitive  vertebrates  then  abounding,  only  a  few  larger 
types  are  shown.  The  dragon-flies  of  that  time  are 
known  to  have  had  a  spread  of  wing  amounting,  in  some 
cases,  to  as  much  as  2  feet.  In  the  foreground,  at  the 
left,  are  representatives  of  the  Cycadofilicales,  some  of 
them  bushy,  and  others  resembling  our  modern  tree  ferns. 
At  the  right  are  dense  thickets  of  Calamites,  the  ancient 
representatives  of  our  modern  scouring  rushes  {Equisetum), 


6l2 


STRUCTURE   AND    LIFE    HTSTORTES 


THE  EVOLUTION  OF  PLANTS  613 

In  the  background,  at  the  left,  are  the  unbranched 
Sigillarias,  and  the  branched  Lepidodendrons.  The  Cor- 
daitales,  which  formed  the  Devonian  forests,  were  not  yet 
extinct,  but  none  is  shown  in  the  figure.  Other  forms, 
ancestors  of  our  modern  conifers  and  angiosperms,  must 
be  imagined  as  hidden  in  the  recesses  of  the  forest. 

526.  Significance  of  the  Fossil  Record. — Before  the 
brilliant  discoveries  in  fossil  botany,  just  outlined,  were 
made,  there  had  been  (as  stated  in  Chapter  XXXVI)  a 
general  tendency  among  botanists  to  consider  the  compar- 
atively simple  moss-plants  as  an  older  type  than  the  fern, 
and  that  either  they  or  their  close  relatives  were  the  ances- 
tors of  Pteridophytes.  As  outlined  in  the  same  chapter, 
the  sporogonium  of  the  moss  was  regarded  as  representing 
the  form  from  which,  by  elaboration  of  vegetative  tissues 
and  organs,  the  sporophyte  of  the  fern  was  derived.  This 
view  was  clearly  expressed  in  1884  by  the  noted  botanist 
Nageli,  who  considered  that  the  sporophyte  of  Pterido- 
phytes was  derived  from  a  moss-like  sporogonium  by  the 
development  of  leafy  branches. 


Fig.  432. — Restoration  of  a  scene  along  a  sluggish  creek  in  Texas  and 
New  Mexico  during  the  late  Carboniferous  (Upper  Pennsylvania)  and 
early  Permian  times.  The  lowlands  of  this  period  doubtless  swarmed 
with  reptiles  such  as  shown  in  the  picture,  and  with  other  animals,  now 
extinct.  Some  specimens  of  the  giant  "dragon-flies"  had  a  spread  of 
wings  of  two  feet.  The  fern-like  trees  and  the  bushy  plants  in  the  fore- 
ground are  Cycadofilicales.  To  the  right  of  the  water  are  wide  stretches 
of  the  huge  scouring  rush  (Calamites);  on  the  left  bank  of  the  stream  are 
the  unbranched  Sigillarias  (still  as  prominent  as  earlier  in  the  coal 
period),  and  on  higher  ground  to  the  left  the  branched  Lepidodendrons. 
One  must  view  this  scene  as  one  of  many  such  landscapes,  with  ever- 
varying  detail,  along  streams  and  inlets.  Cordaites,  which  in  later 
Devonian  time  made  the  first  great  forests  of  which  there  is  record,  is 
still  present,  though  not  shown.  So,  too,  there  are  hidden  in  the  recesses 
of  the  forest  the^  forerunners  of  the  modern  coniferous  types,  as  well  as 
other  forms  destined  to  give  rise  to  the  angiosperms.  (Landscape  from 
Williston,  adapted  from  Neumayr.) 


6 14  STRUCTURE    AND    LIFE    HISTORIES 

A  consideration  of  the  fossil  record,  however,  makes  it 
difficult  to  accept  this  hypothesis.  Not  only  do  we  find, 
in  the  fossil  forms  described  above,  sporophytes  that  do 
not  bear  the  remotest  resemblance  to  the  moss-sporo- 
gonium,  but  fossil  mosses  and  liverworts  have  never  been 
positively  identified  in  either  the  Palaeozoic  or  the  Meso- 
zoic  rocks,  while  the  same  rocks  are  rich  in  fossils  of  such 
advanced  forms  as  the  broad-leaved  sporophytes  of  the 
Cycadofilicales  and  Cycadophytes.  We  must  not,  how- 
ever, hastily  conclude,  from  this  lack  of  evidence,  that 
mosses  and  liverworts  did  not  exist  in  those  early  ages. 
Quite  likely  they  were  present  when  the  Paleozoic  rocks 
were  being  deposited,  though  doubtless  not  represented  by 
the  same  genera,  or  at  least  not  by  the  same  species,  as 
are  now  living. 

527.  Summary  of  Results. — From  what  has  been  said, 
in  this  and  in  Chapter  XXXVI,  we  recognize  that  the 
method  of  evolution  is  to  he  ascertained  chiefly  by  experi- 
ment— by  studying  living  plants  in  action;  but  the 
course  of  evolution  chiefly  by  the  study  of  comparative 
morphology,  with  special  attention  to  fossil  forms.  Other 
points  are  necessary  to  complete  the  history  of  the 
evolution  of  plants;  the  above  paragraphs  give  only  the 
barest  outline  of  the  problem,  for  the  entire  history  is 
much  too  long  and  much  too  difficult  to  be  treated  here. 
To  summarize;  the  facts  now  known  have  led  some 
investigators  to  infer: 

1.  The  origin  of  Angiosperms  from  Cycadophyta  (pro- 
angiosperms) . 

2.  The  origin  of  Cycadophyta  from  Cycadoffiicales. 

3.  The  origin  of  Cycadoffiicales  from  Primoffiices. 

4.  The  origin  of  Filicales  from  Primoffiices. 


THE   EVOLUTION  OF  PLANTS 


6iS 


/Inceatorso/  Pr/mofihces 

Fig.  433. — Genealogical  tree,  showing  the  ancestral  lines  of  the  modern 
plant  orders,  according  to  a  monophyletic  hypothesis.  Full  explanation 
in  the  text.     (Cf.  Fig.  434.) 


6l6  STRUCTURE    AND    LIFE    HISTORIES 

5.  The  origin  of  Cordai tales  from  Primofilices.^ 

6.  The  origin  of  Coniferales  from  Cordaitales. 

An   ancestral   tree   embodying   these  views  is  shown  in 

Fig.  433- 

What  was  the  origin  of  the  Primofilices?  Here,  as 
often  in  every  science,  we  have  to  acknowledge  that 
we  do  not  know;  the  group  is  a  hypothetical  one, 
and  some  investigators  doubt  its  actual  existence 
altogether. 

528.  Other  Views. — (a)  Other  and  equally  competent 
students  of  the  problem  take  exception  to  one  or  more 
of  the  six  points  tabulated  above.  Not  all  of  their  views 
can  here  be  discussed,  but  mention  may  be  made  of 
that  first  elaborated  by  Jeffrey,  of  Harvard  University. 
According  to  this  view  vascular  plants  appear  at  the 
beginning  of  the  fossil  record  as  two  distinct  series,  the 
Lycopsida  and  Pteropsida.  The  Lycopsida,  like  the 
modern  Lycopodiales,  are  characterized  by  the  possession 
of  small  leaves  (a  primitve  character),  and  by  few  spor- 
angia on  the  upper  surface  of  the  leaves.  The  Pteropsida, 
by  contrast,  are  distinguished,  like  the  modern  Filicales, 
by  large  leaves,  having  the  numerous  sporangia  on  the 
lower  surface.  The  two  groups  also  have  well-marked 
anatomical  differences.  The  Lycopsida  reached  their 
greatest  development  in  the  Paleozoic  period,  and  now 
appear  to  be  on  their  way  to  extinction.  The  Pteropsida, 
on  the  other  hand,  although  possessing  many  repre- 
sentatives in  former  geological  ages,  still  maintain 
their  full  vigor,  and  are  considered  by  this  school  of 
paleobotanists   to  be  in  the  direct  ancestral  line  of  our 

^  The  term  Priniofdices,  not  hitherto  used  in  this  text,  refers  to  a  hypo- 
thetical, primitive  fern  stock. 


THE  EVOLUTION  OF  PLANTS 


617 


Ancestors  of  LLjcop^idamd  Pt^ropsida, 


Fig.  434.— Genealogical  tree,  showing  the  ancestral  line  of  the  modern 
plant  orders  according  to  a  polyphyletic  hypothesis.  Full  explanation 
m  the  text.     (Cf.  Fig.  433.)  ^^  ^^  v 


6l8  STRUCTURE    AND    LIFE    HISTORIES 

modern   vascular   plants,    substantially    as    indicated    in 

Fig.  434-^ 

(b)  Greater  precaution  in  drawing  conclusions  from  the 
few  known  facts  has  led  still  other  students  of  fossil  plants 
to  refrain  from  endeavoring  to  connect  up  the  ancestral 
lines,  claiming  that  while  they  may  converge,  indicating  a 
common  ancestry  of  the  known  forms  in  the  geologic  past, 
on  the  other  hand  they  may  not  unite,  or  at  least  may  not 
all  converge  toward  the  same  ancestral  type.  In  other 
words,  it  is  suggested  that  fossil  and  modern  plants  had  a 
poly  genetic  origin  from  the  stage  of  primitive  protoplasm. 
Such  views  are  illustrated  in  Table  VII  (p.  619). 

It  is  seen  from  this  diagram  that  our  modern  ferns  have 
a  long  ancestral  history,  extending  from  the  present  back 
to  early  Palaeozoic  times;  the  same  is  true  of  our  modern 
cycads,  maidenhair  tree  (Ginkgo),  club-mosses  (Lyco- 
podiales),  and  horse-tails  (Equisetales).  The  Coniferales 
may  be  traced  back  into  the  upper  Carboniferous  period, 
while  the  most  highly  developed  of  modern  plants,  the 
Angiosperms,  appear  to  have  come  into  existence  as  late 
as  about  the  middle  of  the  Mesozoic  era,  perchance  as 
recently  as  20  million  years  ago. 

"The  construction  of  a  pedigree  of  the  Vegetable  King- 
dom is  a  pious  desire,  which  will  certainly  not  be  realized 
in  our  time;  all  we  can  hope  to  do  is  to  make  some  very 
small  contributions  to  the  work.  Yet  we  may  at  least 
gather  up  some  fragments  from  past  chapters  in  the  history 
of  plants,  and  extend  our  view  beyond  the  narrow  limits 
of  the  present  epoch,  for  the  flora  now  living  is  after  all 

^  Scott  restricts  the  name  Lycopsida  to  the  Lycopodiales,  and  proposes 
a  third  group,  Sphenopsida,  including  the  Equisetales,  Pseudoborniales, 
Sphenophyllales,  and  Psilotales. 


THE  EVOLUTION  OF  PLANTS 


619 


nothing  but  one  particular  stage  in  the  evolution  of  the 
Vegetable  Kingdom."^ 


Table  VIII 


Ascendancy 


Periods 


Persistence  and  relationship  of 
great  groups 


VII.   Reign  of  Angiosperms 


Tertiary 

Cretaceous 

Comanchian 


VI.  Reign  of  Pro-angiosperms 


Jurassic 
Triassic 
Permian 


V.  Reign  of  Acrogens   (High-   Pennsylvanian 
er   Equisetes.    Lycopods,!  Mississippian 
etc.)  i 


IV.   Reign  of  Gymnospertns    |  Devonian 


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III.   Reign     of     Early    Land 
Plants 


Silurian 


Ordovician 


S-  Actual    Fossil    Land    Plant    rec- 
ord begins 
4.  Primofilices — Early  Equisetes 
3.  Basal   Plant    Complex   with    va- 
riety of  species 


II.   Reign  of  Algae 


Cambrian 
Precambrian 
(Proterozoic) 


Differentiation     of     Dry 

and  Aquatic  Plants 
(Fossil  Algae  abundant) 


Land 


Reign  -of    Primitive    Life 
(Hypothetical) 


Old  P  r  e  c  a  m- 

brian 
(Archeozoic) 


(Fossil  Algae  begin) 
I.  Primitive     Protoplasm 
Unicellular  Life 


and 


In  the  above  table  (after  Wieland),  the  groups  are  to  be  considered  as 
arranged  on  an  unrolled  cylinder,  projected  from  a  hemisphere;  thus  the 
phyletic  lines  are  to  be  pictured  as  diverging  upward  and  the  Cordaitales 
as  coming  between  the  Ginkgoales  and  Filicales,  to  both  of  which  the}^ 
are  related. 


629.  The  Element  of  Geological  Time. — How  many 
years  has  it  taken  for  the  evolution  of  the  higher  Angio- 
sperms— that  is,  from  the  dawn  of  the  fossil  record  in  the 

*  Scott,  D.  H.     "Studies  in  Fossil  Botany,"  p.  3. 


620  STRUCTURE    AND    LIFE    HISTORIES 

Silurian  period  to  the  present?  No  one  knows.  From  a 
study  of  the  thickness  of  rock  strata,  and  a  knowledge  of 
the  probable  time  required  for  the  depositing  of  those 
strata  as  sediment  on  the  floor  of  the  ancient  oceans,  and 
their  elevation  and  denudation  to  their  present  condition 
by  weathering  and  erosion,  geologists  have  been  able  to 
suggest  relative  measures  of  geologic  time.  Paleozoic 
time  is  long,  twice  as  long  as  Mesozoic  time,  and  Meso- 
zoic  time  must  be  at  least  twice  as  long  as  Cenozoic  time. 
The  actual  age  of  the  earth  is,  however,  a  problem  which 
engages  the  attention  of  physicists  as  well  as  geologists. 
Sixty  years  ago  Lord  Kelvin  gave  a  mean  estimate  of 
100,000,000  years.  With  this  estimate  the  geologists, 
Walcott  and  Geikie,  have  nearly  concurred;  but  since  the 
discovery  of  radium  it  has  been  estimated  that  certain 
carboniferous  iron  ores  have  an  age  of  140,000,000  years. 

Figures  of  such  magnitude  convey  but  little  meaning  to 
our  minds;  they  are  too  large  for  us  to  grasp  their  real 
value.  *' Therefore,''  as  Darwin  has  said,  "a  man  should 
examine  for  himself  the  great  piles  of  superimposed  strata, 
and  watch  the  rivulets  bringing  down  mud,  and  the  waves 
wearing  away  the  sea-cliffs,  in  order  to  comprehend  some- 
thing about  the  duration  of  past  time,  the  monuments  of 
which  we  see  all  around  us." 

530.  The  Essence  of  Science. — A  careful  reading  of 
this  book  will  have  led  the  student  to  realize  that  the  un- 
solved problems  of  botany  are  more  numerous  and  quite 
as  interesting  as  those  we  have  solved.  The  essence  of 
science  is  the  endeavor  to  ascertain  by  the  best  method 
that  which  is  most  worth  knowing. 


APPENDIX 


THE  GREAT  GROUPS  OF  PLANTS 


DiTision 


Subdivision 


I.  Thallophyta. 


A.  Algse. 


B.  Fungi. 


II.  Bryophyta. 


III.  Pteridophyta. 


IV.    Calamophyta. 


V,  Lepidophyta. 


VI.   Cycadophyta. 


Class 

r.  Cyanophyceee 

2.  Chlorophyceae 

3.  Phaeophyceae 

4.  Rhodophyceae 

1.  Myxomycetes 

2.  Schizomycetes 

(Bacteria) 

3.  Phycomycetes 

4.  Ascomycetes 

5.  Basidiomycetes 

6.  Fungi  imperfecti 

(life  histories 
imperfectly  known) 


Order 


Hepaticae. 


Musci. 


I.  Eusporangiatae. 


(Ricciales 
Marchantiales 
Jungermanniales 
Anthocerotales 

Andreales 

Sphagnales 

Bryales 

Ophioglossales 

Marratiales 

Isoetales 


2.  Leptosporangiatae. 


■{ 


1.  Sphenophyllineae. .  . 

2.  Equisetineae 

3.  Calamarineae 

1.  Lycopodineae 

2.  Lepidodendrineae —  < 

1.  Cycadofilicineae.  . . . 

2.  Cycadineas 

3.  Bennettitineae 

4.  Cordaitineae 


Filicales 

Marsiliales 

Sphenophyllales 

Equisetales 

Calamarales 

Lycopodiales 

Selaginellales 

Lepidodendrales 

Cycadofilicales 

Cycadales 

Bennettitalei 

Cordaitales 

Ginkgoale& 

Gnetales 


621 


622 


APPENDIX 


A.  Gymno- 
spermae 


VII.  Spermatophyta     ^    ^^^.^ 

sperm  ae 


Pinoideae. 


I.   Monocotyledoneae 


Coniferales 
Tax  ales 
Pandanales 
Naidales 
Graminales 
Arales 
Xyridales 
I  Liliales 
I   Scitamnales 
I   Orchidales 

Dicotyledoneae 32  Orders,  including 

Salicales 
Polygonales 
(a)    Archichlamydeae   |    Ranunculales 


ib) 


Apetalae 
Polypetalae 


Metachlamydeae 
Sympetalae  ( = 
Gamopetalae) 


Rosales 

Violales 

Myrtales 

Umbellales 

Ericales 

Polemoniales 

Plantaginales 

Rubiales 

Campanulales 


INDEX 


Abiogenesis,  89 
Abscission  layer,  loi 
Absorption,  11 
Acacia,  327 
Acer  rubrum,  337 
"Adder's  tongue,"  175,  177 
Adirondack  bog,  580 
Adjustment  to  environment,  189 
Adiantum,  156,  170 

concinnum,  173 
Adnation,  464 
Aerobes,  103 
Aesculus  Hippocastanum,  36,  120, 

121 
Agapanthus  umbellatus,  608 
Agaricus  campestris,  288 
Agassiz,  Louis,  505 
Agathis  australis,  610 
Albugo  Candida,  292 
Alcoholase,  100 
Aldehydase,  79 
Alfalfa  leaf-spot,  295 
Alismacese,  491 
Alisma     Plantago-aquatica,     444, 

492,  493 
Allelomorphs,  555 
Alnus,  83 

Alsatian  clover,  469 
Alsike,  469 
Alternation,  antithetic,  575 

homologous,  575 
Ambrosiaseae,  488 


Amorpha,  467 
Amylase,  80 
Amyloplast,  78,  81 
AnaerobeSj  102,  103 
Anatomy,  4 
Anchorage,  11 
Andreaeales,  193 
Androecium,  435 
Anemone,  462 

Anemonella  thalictroides,  463 
Angiopteris  evecta,  595 
Angiosperms,  408,  446 

ancestors  of,  600 

life-cycle  of  an,  444 
Annulus,  157,  202 
Ant,  fungus-growing,  326 
Anther,  436 
Antheridia,  170,  171 
Antheridiophores,  196 
Anthesis,  472 

Anthoceros,    209,    214,    219,    220, 
341,  356 

fimbriatus,  210 

fusiformis,  209,  214,  215 

laevis,  213 

Pearsoni,  213 

phymatodes,  217 

punctatus,  209 

tuberosus,  217 
Anthocerotales,  210 
Antipodal  cells,  440 
Antiseptic  surgery,  315,  316 
Antitoxin,  314 
Ants,  leaf-cutting,  325 


623 


624 


INDEX 


Apetals,  456,  457 
Apex,  13 
Apical  cell,  195 

Apothecium,  329,  330,  331,  332 
Araceae,  496 
Aralia  spinosa,  27 
Archegonia,  169 
Archegoniates,  222 
Archegoniophores,  197 
Archesporial  cell,  439 
Archichlamydeoe,  456,  457 
Arctium  minus,  486 
Arisaema  triphyllum,  51 
Arrow-leaf,  449 
Ascaris,  186 
Asclepiadaceae,  476 
Asclepias,  476,  477 

syriaca,  475,  478 
Ascomycetes,  98 
Ascophyllum,  352,  364 
Ash,  28 

Aspergillus  niger,  72 
Aspidium  Filix-mas,  158 
Assimilation,  88 
Aster,  24,  25 
Austrian  pine,  416 
Avena  sativa,  298 
Axil,  150 
Azotobacter,  83,  333 


B 


Bacillus  typhosus,  310 
Bacteria,  306 

helpful,  316 

nitrogen-fixing,  318 
Bacteria  and  the  dairy,  316 
Banana,  26 
Barley,  298 

smuts  of,  297 
Barrigona  palm,  130 
Base  of  the  blade,  13 


Bean,  123,  526 

blight,  309 

Pond, 580 
Beech,  336 
Begonia,  179,  347 
Belladonna,  481 
Bennettites,  602 
Betula,  448 

lutea,  II 

populifolia,  108 
Bindweed,  119,  478,  479 
Biogenesis,  89 
Biology,  3 
Biometry,  548 
Birch,  336,  448 

yellow,  II 

white,  108 
Bittersweet,  481 
Black-mosses,  193 
Black  mustard,  464 

walnut,  308 
Bladderwort,  87,  88 
Blade,  13 
Blakeslee,  353 
Blister-blight,  291 
Blister-rust,  pine  tree,  301,  302 
Blodgett,  Frederick  H.,  443 
Blueberries,  451,  452 
Bolley,  Professor,  92 
Boneset,  486 
Boston  fern,  163 

ivy,  144 
Botany,  educational  value  of,  5 

fossil,  5 

relation  to  other  sciences,  3 

systematic,  3 

what  is,  I 
Botrychium,  175,  178,  214 

virginianum,  176 
Bowenia,  ss3>,  4ii 

serrulata,  594 
Bower,  F.  O.,  573 


INDEX 


625 


Brachychiton  diversifolium,  523 
Bracken  fern,  149,  155 
Bracts,  petalody  of,  462 
Brake,  149,  155 
Branches,  sterile  and  fertile,  196 

archegonial,  197 

innovation,  199 
Brassica  alba,  132,  133 

nigra,  464 

oleracea,  530,  569 
Breathing,  respiration  versus,  107 
Breeding  of  resistant  varieties,  313 
Bromeliads,  327 
Brood-buds,  216 
Broom-rape  family,  338 
Brown,  Robert,  16 
Brussels  sprouts,  464 
Bryales,  193 
Bryophyllum,  347 

crenatum,  349 
Bryophytes,  366 
Bryopteris  filicina,  217 
Budding,  98 
Bud-scales,  30 
Bulbils,  163 
Bundles,  fibro-vascular,  36,  61 

structure  of,  65,  66 
Burdock,  116,  486 
Burroughs,  John,  434 
Butter,  June,  316 

Butter-and-eggs,  481,  482,  483-485 
Buttercup,  462 


Cabbage,  464 
Caesalpinioideae,  468 
Calamites,  368,  388,  612 
Calamus,  495 
Calla-lily,  soft  rot  of,  309 
Calla  palustris,  136 
Calorimeter,  106 
40 


Calyptra,  200 
Cambium,  67 

interfascicular,  67 
Camptosorus  rhizophyllus,  162,  164 
Canada  thistle,  486 
Cancer-root,  338 
Cannel  coal,  426 
Capsicum,  481 
Carbohydrates,  71 

seat  of  elaboration  of,  72 
Carbon  cycle,  no,  iii 
Carica  Papaya,  358 
Carpels,  436,  461 
Carya  ovata,  336 
Caryopsis,  493 
Cassia  marilandica,  467,  468 
Castanea  dentata,  294,  296 
Castor-oil  seed.  123 

plant,  63,  66 
Catnip,  479 

Cat-tails,  490,  491,  492 
Cattleya,  329,  500,  501 
Cecropia,  325 
Cell,  15,  16,  17,  18 
Cell-division  and  reproduction,  344 
Cell-sap,  17 
Cell-theory,  15,  20 
Celtis  occidentalis,  292 
Ceratozamia,  333,  411 
Cercis,  468 
Chaff,  495 

Character-units  versus  unit-char- 
acters, 558 
Characters,  inheritance  of  acquired, 

506-509,  565 
Cheese,  316 

Cherries,  black  knot  of,  295 
Chestnut,  294 

American,  296 

bark  disease,  295 

blight,  294 
Chicory,  486,  487 


626 


INDEX 


ChlorophyD,  33,  75 

necessity  for,  321 
Chlorophytum  elatum,  350 
Chloroplasts,  ss 
Chondriosomes,  16,  17,  565 
Chromatin,  16,  186 
Chromosomes,  186 
Cichorium  Intybus,  486,  487 
Cicuta,  472 
Cinnamon  fern,  159 
Cirsium  arvense,  486 
Cissus  laciniata,  342 
Cladonia,  332 
Classification,  3,  365,  538 
Claviceps  purpurea,  293 
Clayton's  fern,  160 
Clethraceae,  473 
Clover,  leaves,  532 

old-maid,  470 

secret  of,  317 
Club-mosses,  376 

little,  382 
Clusia,  328 
Coagulase,  80 
Coal  balls,  596 

Coal-formation,  pollen  and,  424 
Coalescence,  473 
Cocoanuts,  germinating,  499 
Coix  lacrima-Jobi,  446 
Cold  storage,  305 
Colds,  309 
Coleoptile,  494 
Coleorhiza,  494 
Colocasia  antiquorum,  43 
Coloration,  475 
Colpothrinax  Wrightii,  130 
Columella,  200 
Column,  498 
Comandra  umbelkta,  301 

pallida,  301 
Compositae,  483,  488 
Cone-flower,  533 


Conidium,  304 
Coniferae,  416 
Conjugation,  251 

scalariform,  345 
Conopholis  americana,  338 
Convolvulaceae,  478 
Convolvulus,  119 

arvensis,  479 
Coprinus  comatus,  289 
Corallorhiza,  324,  337 
Coral-root,  337 
Cordaitales,  609 
Cordaites,  609,  610 
Coreopsis,  488 
Corn,  297 

Indian,  85,  123,  562 

smut,  299 
Corolla,  435 
Correlation  among  leaves,  143 

within  the  plant,  142 
Cortex,  65,  195 

Corticium  vagum  var.  solani,  291 
Cotyledon,  174,  381 
Coville,  452 
Cow  parsnip,  472 
Crataegus  punctata,  450 
Creation,  special  doctrine  of,  502 
Cronartium  pyriforme,  301 

ribicola,  301 
Crops,  rotation  of,  91 
Cross-fertilization,  173 
Crossing,  increased  vigor  from,  562 
Crossotheca  Hoenginghausi,  593 
Crowfoot,  yellow  water,  460 
Crown  galls,  308 

imperial,  44 
Cruciferae,  463 
Cryptogams,  vascular,  226 
Crysanthemum  leucanthemum,  486 

rust,  313 
Cucumbers,  wilt  of,  309 
Cultures,  pedigreed,  553 


INDEX 


627 


Cupules,  216 
Currants,  301 
Cuscuta,  339,  340,  341 

Cushion,  166 
Cuticle,  35 
Cycadaceae,  83,  331 
Cycadeoidea  dacotensis,  600,  601, 
602,  604 

Dartoni,  605 

Wielandi,  603,  604 
Cycadocephalus  Sewardi,  600 
Cycadofilicales,  592,  598,  612,  613 
Cycads,  390 
Cycas,  83,  411,  431,  447 

circinalis,  391,  401 

media,  395,  397,  398,  40? 

revoluta,  390,  391,  394,  396, 
402 
Cycle,  organic,  70 
Cypripedium,  498 
Cyrtomium  falcatum,  22,  155 
Cystopteris  bulbifera,  164 
Cystopus  candidus,  292 
Cytase,  100 
Cytoplasm,  16 


Dandelion,  116,  486 
Daisy,  white.  486 
Darwin,  Charles,  511 
Darwinism,  510 
De  Candolle,  A.  P.,  92 
Decay   322 
Determiner,  558 
Devvar  flask,  106 
Dextrinase,  79 
Diastase,  86,  100,  102 
Dicotyledons,  453,  456 
Dictyota,  356,  364 
Dififerentiation,  dorso-ventral,  168 
Diflfusion,  55 
of  gases,  55 


Diffusion  of  liquids,  55 
Digestion,  85 
Dionaea  muscipula,  86,  88 
Dioon,  411 

edule,  403 
Diplazium  zelanicum,  154 
Disease  carriers,  310 
Diseases  caused  by  Phycomycetes, 

291 
Diseases,  contagious  and  infectious, 

309 
Dissimilation,  90 
Dodder,  339,  340,  341 
Dog's-tooth  violet,  434,  439 
Dominance,  554 

law  of,  553 
Doubling,  464 
Drosera,  347,  88 

intermedia,  87,  88 

rotundifolia,  348 
Drynaria  meyeniana,  151,  164 


Ecology,  4 

Ectotrophic  micorhizas,  336 

Eel-grass,  348,  349 

Egg-cell,  170 

Egg,  fertilized,  nature  of,  172 

development  of,  1 73 
Eichornia  crassipes,,  Solms   51,  5? 
Elaters,  219,  373 
Elephants,  514 
Elm,  414 

Elymus  virginicus,  293 
Embryo,  174 

growth  of,  174 
Embryogeny,  608 
Embryology,  4 
Enation,  574 
Encephalartos,  411 
Endosmosis,  58 
Endosperm,  400,  430 


628 


INDEX 


Endosperm-nucleus,  440 
Endospores,  99 
Endothia  parasitica,  296 
Environment,  125 

factors  of,  125 

fitness  for,  513 
Enzymes,  79,  86,  94,  100 
Epidermis,  31 
Epiphytes,  326 
Epiphytism,  325,  327 
Equisetales,  368,  618 
Equisetum,  368,  375,  376,  431 

arvense,  368,  370,  371,  374 

debile,  368 

fluviatile,  369 

giganteum,  368 

palustre,  368,  373 

pratense,  368,  371 
Ergot,  293,  295 
Ericaceae,  473 
Eriopus  remotifolius,  226 
Erythronium     americanum,     434, 

437,  439,  455 

albidum,  437 
Etiolation,  136 
Eugenics,  560,  566 
Eupatorium  perfoliatum,  486 
Euthenics,  560 
Evening-primrose,    471,     532,   534 

Lamarck's,  535 

giant,  536 
Evetria  buoliana,  301 
Evolution  and  classification,  539 

early  antagonism  to,  510 

experimental,  520,  535 

inorganic,  503 

method  of,  505 

of  plants,  569 

organic,  504 
Excreta,  theory  of  toxic,  92 
Exosmosis,  58 
Extinction,  factors  of,  589 


Eye-spot,  251 


Factor,  558 

False  beech-drops,  324,  339 

Fascicles,  415 

Fats,  71,  84 

Fawn-lily,  434 

Femaleness,  maleness  and,  353 

Fermentation,  102 

active  agent  in,  99 

alcoholic,  95,  97 

importance  of,  94 

products  of,  96 

relation  of  to  our  daily  lives, 
102 

respiration  and,  112 

significance  of,  loi 
Ferments.  79 
Ferns,  348 

Boston,  528 

eusporangiate,  178,  372 

leptosporangiate,  157 

shield,  158 

walking,  162,  348 
Fertilization,  170,  172,  180 

double,  442 
Fertilization-membrane,  172 
Fever,  typhoid,  310 
Ficus  elastica,  64,  66 
Filament,  436 

Fittest,  survival  of,  191,  515 
Flowers  and  insects,  343 
Flowers,  perfect  and  imperfect,  447 
Foliage-leaf,  types  of,  156 
Fomes  applanatus,  303 
Food  elements,  source  of,  72 
Foods,  kinds  of,  71 
Foot,  174 

Forests,  world's  first,  610 
Fossil,  what  is  a,  578 

record,  gaps  in  the,  587 


INDEX 


629 


Fossombronia  tuberifera,  217 

Fraxinus,  28 

Fritillaria  imperialis,  44 

Fronds,  150 

Fruit,  essentials  of,  451 

Fucus,  352 

Function,  7 

Fungi,  timber-destroying,  303 

Fungus-farms,  325 


Gaertneria,  488 
Galactase,  317 
Gall-mite,  292 
Gametes,  180 

theory  of  purity  of,  555 
Gametophyte,  181 

decline  of,  365 
Gamopetalae,  456 
Gangrene,  316 
Gasteria  nigricans,  38 
Gaussia,  134 
Gaylussacia;  473 
Gemma,  208,  216 
Gene,  558,  559 
Genotype,  561 

Generations,  alternation  of ,  182,  575 
Geography,  plant,  5 
Geological  time,  table  of,  584 
Geotropism,  128 
Geranium,  339 

house,  139 
Germination,  166 
Germ-tube,  166 
GinkgO;  610 
Gleditsia,  468 
Gleichenia  circinata,  157 
Glumes,  495 
Golden  rod,  340 
Goodsell,  John  W.,  310 
Gooseberries,  301 
Grafting,  333 


Grafting,  various  methods  of,  334 

Grain  smut,  297 

Gramineae,  492 

Grape,  downy  mildew  of,  313 

Gravity,    relation    of    roots    and 

stems  to,  126 
Grew,  Nehemiah,  14 
Groups  of  plants,  366 

the  great,  621 
Growing  points,  212 
Growth,  113 

and  nourishment,  123 

differential,  115 

intercalary,  218 

permanent  and  temporary,  121 
Guard-cells,  33,  35 
Gunnera  manicata,  341 
Gymnosperms,  408,  412 

ancestors  of,  609 
Gymnospermy,  408 
Gynoecium,  436 

H 

Hackberry,  292 

Haustoria,  402 

Head,  483 

Hedge  mustard,  190 

Hellriegel,  318 

Henbane,  481 

Hepaticae,  209,  210 

Heracleum  lanatum,  472 

Hercules  club,  27 

Heredity,  541 

experimental  study  of,  549 
Johannsen's  conception  of,  560 
vs,  inheritance,  544 

Heterogamy,  352 

Heterospory,  356,  382 

Hickory,  336 

Hiras6,  404 

Histology,  4 

Hofmeister,  411 


630 


INDEX 


Homology,  28 
Hooke,  Robert,  14 
Horehound,  479 
Hornbeam,  76,  77,  336 
Horsechestnut,  36,  120,  121 
Horsetails,  359,  368 
Hybridizing,  artificial,  552 
Hygiene,  personal,  311 

public,  311 
Hymenophyllum,  351 
Hypanthium,  464 
Hypocotyl,  381 
Hypothecium,  331 


Jungermanniales,  210 
K 

Kalmia  latifolia,  474 
Kingdom,  organic,  8 
Kingdom,  divisions  of  plant,  366 
Knight,  Thomas  Andrew,  128 
Knight's  experiment,  127 
Knowlton,  F.  A.,  595 
Kohlrabi,  464 
Kunze,  599 


Ikeno, 404 

Imbibition,  58 

Incrustation,  578 

Indian  pipe,  323,  339,  473,  476 

Indusium,  155 

Ingen-Housz,  Jan,  73,  74 

Inheritance,  alteration  of,  566 
and  environment,  560 
and  reproduction,  544 
versus  expression,  188,  543 

heredity,  544 
mechanism  of,  565 
what  is,  542 

Innovation-branches,  348 

Internode,  118 

Involucre,  483 

Isogametes,  251 

Isogamy,  352 

Isospores,  380 

Iva,  488 

J 

Jack-in-the-pulpit,  51 
Jeffrey,  426 
Job's  tears,.  446 
Johannsen,  561 
Johnson-grass,  496 
Juglans  nigra,  308 


LabiatcC,  479 

Labium,  479 

Labor,  division  01  physiological,  363 

Lagenostema  Lomaxi,  596 

Lamarck,  506,  509 

Lamella,  middle,  158 

Lang,    ontogenetic    hypothesis  of, 

575 
Larch,  336 
Latex,  476 
Lathraea,  324 
Lathyrus  latifolius,  466 
Layering,  179 
Leaf -base,  13 
Leaf-blade,    internal    anatomy    of, 

31,34 
types  of,  30 

Leaf-fall,  loi,  415 

Leaf -mosaic,  144 

Leaves,  correlation  among,  143 
growth  of,  119 
lungs  of  plants,  103 
relation  of,  to  light,  138 
spore-bearing,  154 
stomach  of  plants,  103 

Leeuwenhoek,  98 

Legume,  318,  468 

Leguminoseae,  466 


INDEX 


631 


Leguminous  crops,  value  of,  83 
Lenticels,  109 
Leonurus  cardiaca,  145 
Lesczyc-Suminski,  Count,  168 
Lichen,  330,  331,  332 

produced     synthetically,    332 
thallose,  329 
Life-cycle,  175 

cytological,  186 

of  an  angiosperm,  444 

of  a  fern,  183 

of  a  pine,  432 

of  Selaginella,  388. 
Life  history,  148 

of  angiosperm,  433 

of  the  pine,  412 
Life  from  life,  all,  89 
Light  on  rate  of  growth,  effect  of, 

13s 
Light-position,  fixed,  141 
Ligule,  382,  384 
Ligulifloras,  484,  488 
Lilium  canadense,  fertilisation  of, 
442 

Martagon,  441 

philadelphicum,  436 
Limb,  487 
Linaria  vulgaris,  481,  482,  483 

484,  485 
Linin,  184 
Linnaeus,  502,  539 
Lip,  479 
Lipase,  86 
Lipoids,  86 

Liriodendron  tuhpifera,  29,  74 
Lister,  Lord,  316 
Little  potatoes,  291 
Live-forever,  31  , 

Lizard's  tail,  32,  459,  460 
Lomaria,  599 

eriopus,  599 
Loxsoma  Cunninghami,  157 


Lunularia,  216 
Lupine,  123,  126,  129 

white,  64,  135 
Lupinus,  123 

albus,  64,  126,  129,  135 
Lycopersicum,  335 

esculutum,  450,  480,  545 
Lycopodiales,  376,  388,  618 
Lycopodium,  376,  378,  431 

cernum,  381 

clavatum,  377 

lucidulum,  377,  382 

obscurum  dendroideum,  377 

phlegmaria,  381 

Selago,  376,  378,  379,  380 
Lycopods,  calamites  and,  368 
Lycopsida,  616,  618 
Lyginodendron    oldhamium,     592, 

593,  596 
Lygodium  japonicum,  157 


M 


Macrozamia,  333,  411,  447 

Moorei,  392,  393,  398,  399,  400 

Magnolia,  606,  607 

Maidenhair  fern,  173 

Maiosis,  185 

Maize,  Mendelian  segregration  in, 
556 

Maleness  and  femaleness,  353 

Malpighi,  Marcello,  14 

Maltase,  79 

Malthus,  513 

Maple,  141 

'       red,  337 

sap,  economic  value  of,  67 
silver,  142 

Marattia  fraxinae,  597 

Marchantia,  348 

Marchantiales,  210 

Marjoram,  wild,  479 


632 


INDEX 


Marsh  grass,  293 
Mary,  typhoid,  310 
Matonia  pectinata,  157 
Medullary  rays,  65 
Megasporangia,  385 
Megaspores,  385 
Megasporophylls,  385 
Melilotus  lutea,  82 
Melons,  wilt  of,  309 
Membrane,  limiting,  17 

nuclear,  186 
Memory,  physiological,  192 
Mendel,  Gregor,  549 
Mendelism,  549 
Meristem,  374 
Mesophyll,  31 
Metabolism,  90 
Metachlamydeae,  456,  473 
Metamorphism,  582 
Micorhiza,  ectotrophic,  336 

endotrophic,  337,  339 
Microcycas,  411 
Microsporangia,  385 
Microspores,  385 
Mid-vein,  37 

Migration  of  plant  diseases,  312 
Migrations,  plant,  586 
Mildew,  downy,  292,  313 

powdery,  292,  295 
Milk,  souring  of,  316 

"certified,"  316 
Milkweed,  475,  476,  477,  478 
Mimosa,  468 

pudica,  122 
Mimosoideae,  468 
Mitosis,  184 
Mohl,  Hugo  von,  15 
Molds,  304 

sexuality  in,  355 

sooty,  295 
Monocotyledons,  489 

and  dicotyledons,  453 


Monocotyledons  origin  of,  571,  608 

types  of,  491 
Monotropa,  473 

Hypopitys,  324 

uniflora,  323,  476 
Monotropaceae,  473 
Moon  worts,  175 
Morchella  esculenta,  289 
Morphology,  4,  147 
Mosses,  true,  193,  204 
Motherwort,  145 
Mountain  laurel,  474 

palm,  134 
Mucor  mucedo,  304 
Mullein,  31,  130 
Multiplication,     vegetative,     163, 

179,  346,  351 
Musa  sapientum,  26 
Musci,  193 
Mushrooms,  288 
Mustard,  white,  50,  133 
Mutation    theory    to    Darwinism, 
relation  of,  537 

test  of,  535 

value  of;  537 
Mutations,  530,  546 
Mutualism,  325,  328 
Muricaceae,  83 
Myrmecophytes,  327 
Myxomycetes,  284,  285,  323 

N 

Nasturtium,  137 
garden,  143 
Nazia  racemosa,  495 
Neck,  169 
Neck-canal,  170 
Neck-canal-cell,  170 
Nephrolepis,  163,  528 
New  Zealand  raspberry,  28 
Nicotiana  Tabacum,  480,  545 
Nightshade,  481 


INDEX 


^33 


Nitrobacter,  83 
Nitrogen  cycle,  318,  319 
Nitrosomonas,  82 
Nodules,  83 
Nomenclature,  4 
Nostoc,  215,  333,  341 
Nourishment,  growth  and,  123 
Nuclear  division,  indirect,  184 
Nucleolus,  16 
Nucleoplasm,  16 
Nucleus,  16 
Nutrition  theory,  91 


Osmosis,  55,  56 

demonstration  of,  57 

importance  of,  59 
Osmunda  cinnamomea,  159 

claytonia,  152,  160 
Ostrich  fern,  114,  178 
Ovary,  436 
Ovules,  436 
Ovum,  170 
Oxalis,  138 

bupleurifolia,  122 
Oxidase,  79 


Oak,  34,  336 
Oats,  smuts  of,  297 
(Enothera,  470 

biennis,  471,  534 

brevistylis,  533 

gigas,  535,  536 

Lamarckiana,  532,  533,  535 
Oleander,  39 
Onagra,  470 
Onagraceae,  470 
Onion,  348 
Onoclea,  172 

struthiopteris,  114,  178 
Ontogeny,  504 
Oosperm,  173 
Operculum,  202 
Ophioglossum,  178,  214 

vulgatum,  175,  177 
Opuntia  Blakeana,  342 
Orange,  295 
Orchidaceae,  497 
Orchids,  327,  500,  501 
Organic  and  inorganic,  69 
Organisms,  8 
Organs,  7 

essential,  448 
Orobanchaceae,  338 


Paleobotany,  5,  578 
Paleogeography,  584 
Palisade,  35 
Palmacese,  495 
Palmetto,  sabal,  498 
Palms,  barragona,  498 

cocoanut,  497 
Pandorina,  405 
Pangens,  565 
Papaw,  359 
Papillionoideae,  467 
Pappus,  487 
Paraphyses,  331,  380 
Parasites,  artificial,  341 

fungal,  342 
Parasitism,  325,  339 

facultative,  343 

obligate,  343 
Parenchyma,  35 
Parmelia  perlata,  330 
Parthenogenesis,  357 
Pasteur,  89,  98,  307 
Pea,  123 

edible,  468,  554 
Peach,  brown  rot  of,  293 

leaf-curl,  293 
P^ar  blight,  309 


634 


INDEX 


Peary,  309 

Peat-mosses,  193 

Pedicle,  197 

Pelargonium,  139,  339 

Pellionia  daveaueana,  78 

Pelories,  482,  485 

Pendulum,  illustration  of  the,  529 

Penicillium  glaucum,  304 

Pennyroyal,  479 

Pepsin,  102 

Perennial  pea,  466 

Pericarp,  493 

Perichastium,  199 

Peridermium  pyriforme,.  301 

Strobi,  301 
Perisperm,  421 
Petalody  of  bracts,  462,  466 

of  stamens,  462,  465,  466 
Petals,  435 

coalescence  of,  462 
Petiole,  13 

structure  of,  35 
Petrifaction.  578 
Phaseolus,  123 

vulgaris,  525 
Phloem,  67 
Photosynthesis,  77 

respiration  and,  no 
Phototropism,  135 
Phyllitis,  156 
Phylogeny,  504,  569 
Phymatodes,  154 
Physalis  alkekengi.  545 
Physcia  stellaris,  329 
Physiology,  4 
Phytogeography,  5 
Phytology,  i 

Phytophthora  infestans,  292 
Phytoptus,  292 
Pine  cone,  420 
Pine,  Kauri,  610 

Scotch,  417,  418,  419 


Pine  white,  421,  422,  423,  428,  429, 
430 

Pine  shoot  moth,  301 
Pine  tree  blister-rust,  301 
Pines,  413 
Pinnae,  156 
Pinnules,  156 
Pinus,  447 

austriaca,  416 

contorta,  301 

ponderosa,  301 

rigida,  301,  415 

Strobus,   415,   421,   422,    423, 
428,  429,  430 

sylvestris,  415,  417,  418,  419 
Pistil,  436 
Pisum;  123 

sativum,  468,  554 
Placenta,  437 

Plant  and  animal  respiration,  112 
Plant-respiration,  105 
Plant    secretions,    economic    value 

of,  90 
Plantago,  37 
Plantain,  37 

Plants  and  animals,  nutrition  of,  70 
Plants  to  man,  relation  of,  2 
Plasma-membrane,  17 
Plasmolysis,  59 

Pleurococcus,  252, 253, 328, 344, 362 
Plum,  brown  rot  of,  293 

pockets,  293 
Plums,  black  knot  of,  295 
Plumule,  443 
Podetia,  331,  332 
Podocarpineae,  83 
Polarity,  364 
Pollen,  abundance  of,  423 

and  coal-formation,  424 

immediate  effect  of,  451 

shedding  of,  424 
Pollen-chamber,  400     • 


INDEX 


63s 


Pollen-grain,  401,  436 
Pollen-sacs,  436 
Pollination,  401,  427 
Pollinium,  477,  501 
Polypetalse,  456 
Polypodium,  154,  156,  iS^ 
irioides,  154 

punctatum,  155 
Polytrichum  commune,  206 
Populus,  457 

Porella  navicularis,  211,  212 
Potato,  335»  347,  480 
blight,  313 
rot,  292 
tuber,  81 
Potatoes,  little,  291 
Potonie,  598 
Pressure,  osmotic,  58 

and  growth,  osmotic,  113 
Priestley,  Joseph,  73,  74 
PrimofiUces,  616 
Proangiosperms,  606 
Propagation,  180 
Prophylaxis,  310 
Protease,  79,  86 
Protein,  16 
Proteins,  71 

manufacture  of,  84 
Prothallus,  166,  167 
Protonema,  165,  166 
Protoplasm,  14,  15 

properties  of,  19 
Protoplast,  16 
Pseudoborniales,  618 
Pseudomonas    radicicola,    82,    83, 

318,  332 
Pseudopodium,  200,  201 
Psilotales,  618 
Pteridophytes,  366,  388 
Pteridosperms,  592 

significance  of,  598 
Pteris,  156 


Pteris  aquilina.  149,  iS5 

longifolia,  153 
Pteropsida,  616 
Ptyalin,  100 
Puccinia  graminis,  299 
Punk,  303 

Purity  of  gametes,  theory  of,  555 
Pussy-willow,  458 
Putrefaction,  319 
Pyrenoids,  353 
Pyrolaceae,  473 
Pythium  de  Baryanum,  291 


Quarantine,  311 
Quercus,  336 

novimexicana,  34 
Qu6telet.  524 
Qu6telet*s  curve,  525,  526 

R 

Radicle,  381,  464 
Ranunculacese,  459 
Ranunculus,  459,  460,  461,  462 

aquatilis,  507 
Rattan,  495 
Rattlesnake  fern,  176 
Ray,  John,  453 
Recptacle,  155 
Redi,  89 
Reduction,  183,  184 

division,  185,  187 
Regeneration,  205 
Reproduction,  asexual,  179 

by  spores,  180 

cell-division  and,  344 

essence  of  all,  180 

inheritance  and,  544 

sexual,  179,  180 


636 


INDEX 


Respiration,  105,  iii 

and  fermentation,  112 

and  photosynthesis,  no 

versus  breathing,  107 
Response,  stimulus  and,  125 
Rex  begonia,  347,  447 
Rhizoctonia,  291,  293 
Rhizomes,  150 
Rhubarb;  116 
Ribes,  301 
Riccia,  222 

fluitans,  209 

life  history  of,  224 

natans,  209 

trichocarpa,  222 
Ricciales,  210 
Ricciocarpus,  223 
Ricinus,  123 

communis,  63,  66 
Robinia,  467 
Rock  strata,  classification  of,  583 

Root,  ID 

elongation'of,  117 
Root-cap,  50 
Root-hairs,  11,  53,  54>  58 

and  soil,  relation  between,  52 

importance  of,  49 

location  of,  49 

structure  of,  51 
Root-nodules,  83 

of  Cycas  revolutia,  333 
Root-stocks,  150 
Rosa  Carolina,  464 

rugosa,  450 
Rosacese,  464 
Rose,  465 

wild,  466 
Rosette,  40 
Rostellum,  501 
Rotation  of  crops,  318 
Rubber-plant,  64,  66 
Rubiaceae,  463 


Rubus  australis,  28 
Rudbeckia,  533 
Rue  anemone.,  463 
Runners,  164,  350 
Rye,  cultivated,  293 
wild,  293 


Saccharomyces,  98 
Sage,  479 
Sagittaria,  449 
Sago  palm,  390 
Salix,  12,  346,  457,  458 

babylonica,  457 

discolor,  457 
Salvia,  480 
Salvinia,  341 
Salicaceae,  457 
Saltation,  orthogenetic,  528 
Sanitary  theory,  92 
Sanitation,  311 
Sap,  nuclear,  184 
Sapium  sebiferum,  84 
Saprophytes,  fungus,  322 
Saprophytism,  321,  322 
Saururaceae,  459 
Saururus  cernuus,  32,  459,  460 
Saussure,  Nicolas  Theodore  de,  74 
Savory,  479 
Schleiden,  15 
Schwann,  15,  94 
Science,  essence  of,  620 
Scion,  335 
Scott,  D.  H.,  619 
Scrophulariaceae,  481 
Scutellum,  494 
Secale  cereale,  293 
Secretions,  19 
Sedum,  31 

Seed,  essentials  of,  453 
'Seed-coat,  397,  453 


INDEX 


637 


Segregation,  law  of,  555 

Mendelian,  554,  556 
Selaginella,  382,  384,  431 

amoena,  383 

Kraussiana,  387 

life-cycle  of,  388 

Martensii,  385 

Watsoniana,  384 

Wildenovii,  383 
Selaginellales,  382.  388 
Selection,  natural,  191 
Selfing,  560 
Self-pruning,  180 
Semi-parasitism,  341 
Sempervivum  tabulaeforme,  41 

tectorum,  350 
Senebier,  Jean,  74 
Senna,  wild,  467 
Sensitive  plant,  122 
Sepals,  435 
Serum-therapy,  314 
Seta,  204 
Sex,  determination  of,  358 

meaning  of,  360 

problem  of,  in  plants,  344 
Sex-organs,  differentiation  of,  356 
Sexual  characters,  secondary,  358 
Sexuality  in  molds,  353 

in  Spirogyra,  355 
Shaggy-mane  mushroom,  289 
Shelf-fungus,  303 
Shoot,  9 

Sigillarias,  612,  613 
Silique,  463 
Siphonogamy,  406 
Skein,  nuclear,  186 
Skunk  cabbage,  499,  500 
Solanaceae,  480 
Solanum,  335 

Dulcamara,  481 

integrifolium,  545 

nigrum,  545 


Solanum  tuberosum,  347,  480 
Solidago  ulmifolia,  340 
Soredia,  330 
Sorghum  halpense,  496 
Sorus,  154 
Soy  bean,  317 
Spartina,  293 
Spathyema  fcetida,  496 
Species,  elementary,  530 

origin  of,  510,  587 
Spermatophytes,  408 
Sphaerothica  phytoptophyla,  292 
Sphagnum,  193,  194,  200,  202,  204, 
348 

acutifolium,  198 

cuspidatum,  199 

cymbifolium,  195 

life-history  of,  203 

squarrosum,  198 
Sphagnales,  193 

Sphenophyllum  cuneifolium,  372 
Sphenophyllales,  618 
Sphenopsida,  618 
Sphenopteris  Hoeninghausi,  592 
Spike,  459,  495 
Spikelets,  495 

Spirogyra,  344,  345,  353,  355,  364 
Sporangiophores,  371 
Sporangium,  155,  156,  157 
Spore-mother-cells,  158 
Spores,  155 

dispersal  of,  164 

germination  of,  165 

swarm-,  251 
Sporogonium,  201 
Sporophylls,  156,  159 
Sporophyte,  181,  201 

development  of,  364 

evolution  of,  574 
Spruce,  140 
Staff-tree,  481 
Stalk-cell,  428 


638 


INDEX 


Stamens,  417 
Stangeria,  411 

eriopus,  598 

paradoxa,  598 
Starch-making,  77 

steps  in,  79 
Stems,  elongation  of.  118 

exogenous,  65 
Sterilization,  162 

progressive,  432 
Stigma,  437 

Stimulus  and  response,  125 
Stock,  335 

Stolons,  164,  349,  350 
Stoma,  33,  35 

Stomata  and  gaseous  exchange,  108 
Strata,  classification  of  rock,  583 
Stratification,  582 
Strawberries,  348 
Strobilus,  371 

Struggle  for  existence,  190,  513,  515 
Style,  437 
Sugar-beet,  68 
Sugar-cane,  68 
Sugar-maple,  336 
Suminski,  Count  Lesczyc,  168 
Sundew,  87,  88,  347 
Sunlight,  importance  of,  75 
Survival  of  the  fittest,  191,  515 
Suspensor  380 
Swarm-spores,  251 
Sweet  clover,  yellow,  82 
Sweet  pea,  470 
Symbionts,  318 
Symbiosis,  214, 321,  324,  325 

social,  325 
Symmetry,  bilateral,  167 
Sympetalae,  456,  473 
Symplocarpus  foetidus,   496,   499, 

500 
Synangia,  601 
Synergids,  440 


Tap  root,  10 

Taraxacum,  486 

Taxonomy,  4 

Tectoria  cicutaria,  161,  163 

Tetrad,  158 

Tetrad-divisions,  158,  187 

Tetraphis,  207 

Thallophytes,  366 

Thallus,  167 

Thecium,  330,  331 

Thyme,  479 

Thyrsopteris  elegans,  157 

Tillandsia,  326 

Tissue- tension,  117 

Toadflax,  481,  482,  483,  484,  485 

Toadstool,  288 

Tobacco,  480 

Todea  barbara,  157 

Tomato,  335,  450,  480 

Torus,  464 

Tour,  Cagniard  de  la,  98 

Toxins,  313 

Trachymyrmex  obscurior,  326 

Transpiration,  advantages  of,  40 

control  of,  38 

cuticular,  38 

lifting  power  of,  42 

stomatal,  38 
Tree  ferns,  150 
Tree,  genealogical,  577 
Trifolium  hybridum,  469 
Triticum,  298 

vulgare,  494 
TroUius,  462 

Tropaeolum  majus,  137,  143 
Trunk,  412 

excurrent,  413 

deUquescent,  414 
Trypsin,  102 
Tube-cell,  423 


INDEX 


639 


Tubers,  217 

Tubuliflorae,  488 

Tulip  bulb,  29 

Tulip-tree,  29,  74 

Turgor,  58,  114 

Turpin,  98 

Tyndall,  89 

Type  genus,  491 

Typhaceae,  491 

Typha  latifolia,  490,  491,  492 

angustifolia,  490 
Typhoid  fever,  310 

Mary,  310 

U 

Ulmus  americana,  414 
Ulothrix,  352 

zonata,  354 
Ultricularia,  87,  88 
Umbelliferse,  471 
Umbels,  471 
Unconformity,  583 
Unit-c  h  a  r  a  c  t  ers,  character-units 

versus,  558 
Ustilago,  298 

Avenae,  298 

maydis,  299 

Tritici,  298 

Zeas,  298 

V 

Vaccination,  313,  314 
Vacciniaceae,  473 
Vaccinium,  451,  473 

corymbosum,  452 
Vacuoles,  17 
Vallisneria  spiralis,  349 
Van  Tieghem,  598 
Variability,  fluctuating.  525 
Variation,  189,  512 

continuous,  522 

discontinuous,  527 


Variation,  fluctuating,  and  inheri- 
tance, 527 
Vascular  plants,  153 
Vaucheria,  364 

terrestris,  356 
Venter,  169 
Ventral-canal  cell,  170 
Venus's  flytrap,  86,  88 
Verbascum  Thapsus,  31,  130 
Verbena  ciliata,  33 
Vernation,  circinate,  152 
Vinegar,  317 
Vivum  e  vivo,  omne,  89 
Von  Mohl,  Hugo,  15 
Vries,  Hugo  de,  520,  521 

W 

Wall-cell,  428 
Water  and  the  soil,  54 
Water  crowfoot,  460 
Water  hyacinth,  51,  52 
Water  importance  of,  47 

plantain,  493 

requirement,  relative,  47 
Webber,  404 
Wheat,  smuts  of,  297 
Wheat-sickness,  92 
White  pine  blister  rust,  302 
Whorls,  412 

Wild  cabbage,  horticultural  varie- 
ties of,  531 
Wieland,  597 
Wielandiella,  608 
Williamsonia,  602,  608 
Willow,  12,  117,  346,  457,  458 

weeping,  457 
Wilt  disease  of  cotton  and  water- 
melon, 295 

cucumbers  and  melons,  309 
Wings,  166 

Witches'  brooms,  292,293 
Wood  lily,  436 


640  INDEX 

Woodwardia  orientalis,  351,  352 
X 


Yeast  what  is,  98 
Yucca.  62 


A'-chromosome,  359,  360 
Xeno-parasitism,  343 
Xylem,  65 


Yeast,  95,  98,  306 


Zamia,  335,  393,  411,  610 

floridana,  404,  407 
Zea  Mays,  62, 85, 123,  299,  562,  563 
Zygospores,  345 
Zygote,  180 


u   ^ 


